Water contact optics have existed in various forms since film cameras however in the last 20 years with the advent of digital compact cameras the emphasis has been on wet lenses.
A wet lens would be attached to the housing of the camera and be in contact with water.
Inon has been for a long time the champion of wet lenses but since 2015 the top performer has been Nauticam since the introduction of the WWL-1 which I reviewed in this article still using a compact camera. Originally wet lenses were only designed for small sensor systems however the WWL-1 was the first wet optic compatible with the Sony full frame E-mount system. The issue with large systems is that as the lens gets bigger the wet lens gets bigger at some point you need to move to a dry mount to avoid vignetting but the optical design remains unchanged.
Dry mount wet lenses existed long before Nauticam efforts; the most notable was the FIX dry mount for the canon G16 series.
How does a wet lens work?
All wet wide angle lenses are built as afocal (focal length equal to infinity) system with magnification smaller than 1 this means things look smaller than they are when you apply a wet lens on top of the camera lens.
This has a significant benefit for all lenses that have a minimum working distance that is not compatible with a dome port, which requires lenses to focus really close.
With an afocal lens designed similar to a laser beam extender: things look further away and therefore the system is able to focus right on the surface of the wet lens.
Historically wet lenses were not corrected for distortion and, as result of having this demagnified effect, they also have barrel distortion which is commonly accepted in underwater photography.
When we look at wet lens specifications, we are given the magnification of the lens alone for example 0.36x or 0.57x. This does not mean that if your lens is 28mm it becomes 28×0.36=10.08mm during the process.
What it means is that if something is at 1 metre from the wet lens this will indeed look as if it was 2.8 metres and therefore the field of view will be larger. This is a significant benefit if you compare to the same 28mm behind a flat port which is the starting point of the compact camera market those optics have been originally designed for.
When you combine the wet lens with your camera lens the demagnification effect may be affected by the distance between the two lenses, the length of the optical path inside the camera lens and body, and inside the wet lens.
As you work at distances that are much longer than the physical size of your system the magnification factor becomes a more accurate approximation, however as you get very very close to the point of touching the class the magnification actually increases and the effect of the wet lenses decreases.
WWL-1 on Sony 28/2
Wet lens operating at very close range the depth of field has dropped far away from the notional 130 degrees diagonal but the image remains very sharp.
And this brings to a fundamental difference and key takeaway: the field of view of a water contact system, composed by a camera system and a wet lens, changes with the subject distance.
Another important point is that due to the design, a system with a wet lens can indeed focus at infinity which is totally different from a dome port system, a system with a wet optic based on focal design does not require the lens to work at close focus but in most case will allow the lens to work within the specified MOD even when subjects are actually closer.
Why a water contact optic may work better than a lens in a dome?
We have read about the marketing that states that water contact optic A has a benefit of N stops in the corners compared to a rectilinear lens of the same field of view.
What does this mean and is it true? First, the majority of rectilinear lenses are unable to offer the same diagonal field or view of a wet lens combined with a longer optic so this comparison is a bit difficult. Second majority of dome ports on the markets are limited to 14mm widest focal length so a comparison to match the field of view is not entirely possible.
If your rectilinear lens has very poor close focus performance, which is what happens in a dome where a lens works in a distance range around max 0.5 metres, and you compare it with a standard lens combined with a water contact optic which helps the lens to focus further away, you will find that on a resolution target the water contact optic may indeed offer much better sharpness as you move from the centre to the edges of the frame.
This is not an absolute truth as there are some modern mirrorless lenses that focus very close and in fact match or even beat an average lens combined with a water contact optic.
Sony 1635 F11
The Sony 16-35GMII in the example above offers superb corner sharpness and in fact wider field view at close range than the wet lens despite a smaller nominal field of view. This is due to the fact that the dome optic preserves the field of view at all shooting distances.
The 16-35GMII however costs more than the Sony 28-60mm and WWL-1 combined without accounting for the cost of the dome!
If we consider other solutions like the WACP-1 the dome solution costs become comparable or even lower.
Limitations of Water Contact Optics
Allowing the lens to focus further away than the subject may allow the lens to work in a better performing zone however this also comes with some drawbacks.
In situations where the subject is flat there are no issues with a water contact afocal system however reality is not made of flat subjects.
In this example we have our target at 1 metre and the reef at 0.5 metres while at 2 metres there is some background of interest. The examples are based on full frame system with a distance to the front of the wet lens of 19cm.
If we model a water contact optic we see that the aperture required to cover the depth of field while focussing on the target is f/8. This is based on the sensor virtual distances on the graphic.
However the f/stop goes to f/13 if our focus is very close or very far.
Take into account that in real like the magnification will be higher than 0.36 and therefore the values computed are best case scenarios.
With this type of adapter both focus position and field of view are based on the virtual image.
This is a classic example that shows the effect.
Moray on Red
The moray eel is at around 1.5 meters however the camera thinks the target is 5 meters away as result even shooting at f/11 results in soft edges and lack of depth of field in front of the subject.
If we do the same simulation with a lens housed in a 9” dome the results are very different due to dome port compression.
Remember that for a dome virtual image drives the focus position however field of view is based on real distances.
Example shot in a similar situation with a Canon 8-15 and teleconverter at f/11 the entire frame including the parts ahead of the coral block are sharp. The camera focus position is 27cm due to the dome port optic visual image, the dome size used is 69mm radius.
We can see that in terms of depth of field the dome system is much more effective despite the subject distance is very near, although this will be penalised by other aberrations and therefore more likely work at around f/8 still this is a benefit especially when we look at closer targets which don’t require the same small apertures of the wet lens system.
This other example with teleconverter at f/8 focal length 21mm shows decent depth of field even in the near edge. At f/11 this would have been sharp throughtout
Red Raindrops
Therefore the water contact optic typically requires smaller apertures compared to a shorter lens of comparable field of view on the horizontal axis within a dome solution.
This reef scene shows a classic situation where you need to stop down the lens a lot to have everything in focus here shot at f/16 so that the coral in front of the fish is sharp.
Sway
With f numbers going past f/11 there is a sensible drop in resolution of the overall system and therefore the water contact optic appears more suitable to situations where the main subject is ahead of anything else in the frame, for example schooling fish.
Staircase
A school of snappers in Malpelo makes the perfect target for a wet optic as the reef is behind and everything looks sharp across the frame at f/11.
Reading the exif of this image we see that the camera thinks the target was at 8.05 meters in reality it was less than 3 meter away.
Fish Shape
Summary
Water contact optic allow lenses to work in a more comfortable focus range, this is very advantegrous if we compared it to a flat port or to a wide lens that does not work well at close range.
However wet wide angle lenses are not suitable to all scenarios, in particular due to the issues of depth of field and also lower effect at close range are not particularly suited to close focus work without a significant loss of resolution on modern full frame mirrorless cameras.
If your objective is to work at close range and have reef scenes that are not clean with elements before and after the focus target a fisheye with teleconverter may offer a more fitting solution than a 0.36x wet lens with a standard zoom because it does not loose magnification and ends up in comparable aperture numbers to achieve a similar depth of field.
For targets at far distance shorter focal lenght, in our case half (14mm vs 28mm) will offer superior depth of field because of the dome port optics however will need to be stopped down to fix other aberrations yet unlikely to require aperture smaller than f/11 or in some cases even f/8 and therefore providing a potential increase also in light gathering and resolution.
Ultimately on full frame cameras wet optics are not a silver bullet and solution for all shooting scenarios but just another option to be used for example to reduce drag, consider the effort of finning with a small wwl-1 compared to a 9″ dome port in 3 knots current.
As I wrote in a previous article this lens has several strengths
Close minimum focus distance (19 cm wide – 26 cm tele)
Lens does not extend when zooming
Reasonably compact (99 mm and 420 grams)
Good sharpness at the edges from f/4 onwards
Low cost compared to other Sony lenses.
The lens will cost you $799 vs $2,299 of the Sony 16-35 GMII which is the best lens in this class however the price difference will convince most people especially those only using the lens underwater that the Tamron is the way to go.
Parts for Nauticam Housings
The Nauticam 18809 wide angle dome port is not a classic dome but has design without a flat base. The port has 11cm radius of curvature and is 85mm deep 180mm wide this means the entrance pupil needs to be 25mm behind the extension.
Nauticam 18809 180mm Wide angle dome port
Nauticam recommended extension is 40mm when combined with the 35.5mm N120 to N120 port adapter. Due to the shape of the lens this cannot be used with the N100 port as the zoom is close to the front of the lens.
Tamron 17-28mm with Nauticam zoom gear
Underwater housing manufactures unfortunately do not apply any science to the selection of domes and extension for a lens but out of pure coincidence the 40mm extension ring is what this lens requires.
Tamron 17-28mm 35.5 adapter and 40mm extension
With the 40mm extension the glass port will be exactly 11 cm from the entrance pupil and focus right on the surface.
The Nauticam parts will set you at $2,284 for the gear, extension and wide angle port.
Pool Session
I had already shot the Tamron in the murky waters of my local pool so I went to Luton that has a better filtration system and started with my usual shots.
I took shots from f/2.8 to f/22 obviously f/2.8 and f/4 are purely academic but decent results are obtained from f/5.6.
Tamron 17 2.8 Close
At F/2.8 most of the area outside centre is blurred.
Tamron 17 4.0 Close
By f/4 we have a substantial improvement.
Tamron 17 5.6 Close
At f/5.6 the lens is better than most already considering the very close shooting distance. Unfortunately at this stage I picked up a bit of debris on the dome and did not realise…
Tamron 17 8.0 Close
f/8 is very good and this is your default for shots that are not close when edges are not important.
Tamron 17 11 Close
f/11 is probably the best overall compromise between edges and centre.
Tamron 17 16 Close
By f/16 depth of field keeps everything in focus however the lens has dropped in the centre.
Tamon 17 22 Close
f/22 gives you a consistent frame but with evident resolution loss.
All the shots above have distortion correction deactivated.
I then went and shoot a tile wall to see how straight is the lens here lens correction is applied.
Tamron 17 5.6 Wall
At f/5.6 shooting from 1.8 meters performance is excellent.
Tamron 17 8.0 Wall
f/8 is even better across the frame and is your default if depth if field is not essential.
Tamron 18 11 Wall
f/11 is great
Tamron 17 16 Wall
f/16 and f/22 give consistent sharpness as expected again those apertures are normally not necessary.
Tamron 17 22 Wall
Shooting people with the Tamron
One of the things you do with a rectilinear lens is to shoot straight lines and correct proportion people and wreck interiors for example. The inside of the pool lends itself well to this.
Tamron 17 2 divers 8
With subject not close the lens has a great pop and rendering at f/8
Tamron 17 side 8
Even closer subject with not far background look great.
Tamron 17 diver 8
If you need the background to be sharper you can stop down.
Ascent 11
Again considering that even WWL-1 and WACPs really need f/11 this looks terrific.
Knee down 14
A final example shows that f/14 is enough to give you the depth of field you need when the subject is not too close.
Conclusion
There is no doubt that if you are in the market for a rectilinear wide angle and you are budget conscious this is the lens to get full stop!
Warning this is an extremely technical article that I have written on request. If you are not familiar with optics, geometry, housings do not attempt to perform a calculation by yourself and rely on expert advice.
Background
The physics of dome ports are not new to underwater practitioners although not many people understand the formulas, it is well accepted that there is a correct way to size and position a dome port in order to optimise optical performance of a lens inside an underwater housing. I do not want to repeat the theory here but if you feel you need a refresher the excellent articles from the now passed David Knight and specifically the piece on dome port theory will be useful. For the purpose of this article I will consider only underwater imaging, split shots and over and under have different considerations and will be addressed separately in due course.
Practical Implications
For our purposes, what is interesting is that a dome port is able to restore the lens air field of view when the camera and lens are inside a housing. The theory says that this happens when the centre of the dome lies on the lens entrance pupil. But what happens if it does not?
Jeremy Somerville has created a number of visualisers that although not totally correct give a good idea of the issues involved. In particular the positioning of the dome port is something you may want to check. In short if the dome is not correctly positioned we lose field of view as result of distortion and increase the amount of chromatic aberrations.
We also have to consider that the dome port being a single element lens has also issues of field of curvature and spherical aberrations which are additional to any considerations on positioning and require the user to stop down the lens to reduce the side effect. Those side effects are exacerbated when the dome is not correctly positioned to the point they cannot be corrected no matter how much you stop down the lens.
Choosing appropriate wide angle lenses
Minimum Focus Distance
One of the key takeaways of dome port theory is that if your lens is not able to focus close it may not work at all inside a dome, which in turn means your dome starts to become bigger and bigger to allow your lens to focus or you need to introduce close up lenses which further deteriorate optical quality.
More compact set ups and smaller domes require lenses that can focus close. In addition, due to the dome port optics, infinity focus will be reached at 3x the dome radius from the dome surface: your lens will work to a maximum focus distance well under one metre and closer to half a metre. This is a challenge for wide angle lenses that are designed for landscape and not usually optimised for close focus. One assumption that you cannot make is that a lens that is great for topside use will perform equally well behind a dome, or even more interesting a lens that is small and compact may require a quite sizable dome to work properly underwater which negates the size benefit to start with.
The dead Zone
The dead zone is where the camera cannot focus because our subject is too close. Our objective is to place the dead zone inside not outside our dome so that we can maximise the range we can use for imaging. It is not an issue if the dome radius is so big that the focus area falls well inside the dome, in fact it may be an advantage, but if the camera focus distance is outside the dome we are eating away useful range and at the point where the focus distance is so far that is outside the dome infinity point the camera will not focus at all.
By choosing a lens that can focus very close we accomplish two objectives:
We reduce the size of the dome required
We maximise the focus range that can be used.
I prefer lenses that have a minimum working distance around 20cm, and avoid anything that focuses from 25cm and beyond, this ensures good image quality and reasonably compact set ups.
Prime vs Zoom
Prime lenses have a fixed entrance pupil this means that once the dome is sized and positioned your job is done. Zoom lenses instead change in size or move the entrance pupil to accommodate changes in the field of view. This is bothersome as it means that if you determine your dome parameters at wide end this may not be correct at tele end. In addition as the angle of view is being reduced the curved surface of the dome will start looking more and more flat. This is a challenge but not one we need to address, as seen in the flat port theory lenses that are longer than 35mm suffer less from chromatic aberrations, therefore for our purposes we will treat zoom lenses like a prime lens whose focal length is the shortest our zoom can manage, i.e. the wide end of the zoom. At the tele end the dome with a zoom lens will look like a flat port but still have some benefit over it in terms of aberrations.
Zoom factor
Although we said we will consider the zoom lens as a wide tele, lenses with a zoom ratio much bigger than 2x will most definitely be problematic. This is the reason why zoom lenses with conservative ratios like a classic 16-35mm are bound to perform overall better than say a 20-70mm lens. Lenses in the classic 24-70mm or 28-75mm range tend to have less problems because they are not that wide to start with and generally work well as long as they focus close, otherwise they will require larger domes.
Example Cases
I have good experience after one year on e-mount and therefore I am going to list a few examples of lenses that are excellent topside quality but are bound to work not so well underwater as well as other lenses that have good potential at different price points.
Lenses requiring large domes for optimal performance
Sony 12-24mm F.2.8 GM – a high quality super wide zoom lens that is great for topside use. It has a minimum working distance of 28cm and is 13.7cm long. This lens will likely require a dome port with a radius in excess of 14.3 cm and a field of view of 122 degrees. A port like this is not standard on the market.
Sony 20-70mm F4 G – a versatile topside zoom with extensive zoom. It has a minimum working distance of 30 cm and is only 9.9 cm long this lens is likely to require a dome port over 20 cm in radius to perform at its best.
High Potential Lenses
Tamron 17-28mm F2.8 – A lens that is cost effective and sharp with a limited zoom range. It can focus as close as 19cm and with a size of 9.9cm will require a dome of just over 9cm to have the focus range inside the dome.
Sony 16-35mm F2.8 GM2 – An excellent topside lens that has a good zoom range and can focus at 22cm. With a physical size reaching 12cm this lens is likely to work with domes that are not excessively large.
Sony 20mm F1.8 G – An amazing low light lens that can focus at 19 cm. With a physical size of 9cm this lens is likely to work with relatively small domes and produce outstanding image quality.
Comparison at equal field of view and different working distance.
The graphic above illustrates how two lenses with equal field of view displayed in solid green require different dome radii depending on the minimum operating distance. The small inverse triangle is the area inside the lens up to the focal plane.
Lens1 will require the smaller dome so that the area not in focus falls inside the dome, if a larger dome is used this simply expands the focus range into the water proportionally to the increased dome radius. A lens with the same field of view but longer MOD2 will require a larger dome to ensure the area out of focus is inside the dome. A smaller dome can be used however the dead non focus area now moves into the water. As the infinity point is still set at 3x the dome radius from the surface using this smaller dome means less focus range can be used by the camera. Using too small domes deteriorates image quality because the compressed focus range has an impact on the overall image resolution.
Locating the Lens Entrance Pupil
In order to properly position the dome port we need to determine where the entrance pupil of our lens is. There are at least 4 methods that can be used to locate the entrance pupil of the lens.
Method 1 Look into the lens
It makes me smile when you read: locating the entrance pupil is easy just look into the lens and see where the aperture is. I do not find this easy at all, first lenses are increasingly complex in construction and second how do you place depth of the aperture correctly even if you can see it? The error margin of this method is very high.
Method 2 Non Parallax Point
If you are into panorama photography you know that you can locate the non parallax point which is the lens entrance pupil using a slide mount and a specific set of targets.
A demonstration of this method is beyond this write up however if you want to go deeper into this this article should help you. This method has a good level of precision and panotools maintains an entrance pupil database for many DSLR lenses.
Method 3 Trigonometry
Once you know the lens field of view you can use various filter rings to determine the thickness where vignetting occurs. At that point you can simply calculate the distance from the edge of the entrance pupil by taking the ratio between the lens radius and the tangent of the angle of view. This gives good precision and does not require anything else than the lens itself and a few filters but can be approximated also for a lens you do not own using standard roundings.
Method 4 Lens Design
There are some websites that have lens design drawings directly from patents. This will give you the exact location of the entrance pupil from the image plane and from the lens mount.
I use the site maintained by Bill Claff called the Optical Bench Hub. Unfortunately the database is not complete, some specific brands designs are scarce. The benefit of this method is that you can use it to make calculations before you buy the lens and it is 100% accurate.
Entrance Pupil Determination – Practical Examples
Case 1: Lens Design Available
Sony 20-70mm F4 G lens is a very versatile zoom, whe wide angle 20mm is sufficient for many situations and the tele end of 70mm good for close up work on land. This lens makes a good candidate for underwater use in terms of angle of view however it has a minimum operating distance of 30 cm which is far from ideal.
We locate the lens design on the Optical Bench Hub here.
The important parameters are I distance from the edge of the lens to the image plane (sensor) which is 115.04mm and P distance of the entrance pupil from the lens front.
The difference I – P = 91.79mm still accounts from the flange distance. Taking that out we get 73.79mm from the lens mount.
The lens has a minimum working distance of 300mm. If we subtract the entrance pupil distance from the image plane of 91.79mm we determine a minimum dome radius of 208.21mm which is rather large and in fact not available if not as a custom product.
Of course we can still go ahead and use a smaller dome radius however all the range between the MOD and the edge of the dome will be wasted and not produce an image in focus.
Sony 20mm F1.8 G lens is the modern equivalent of a Nikkor UW15 giving a field of view that is acceptable for most uses and excellent image quality.
The lens has a MOD of just 19cm the distance I-P=80.91 a dome with a radius of 11cm will contain the entire non focus area. The entrance pupil is 62.91mm from the lens mount.
Case 2: Lens Design Not Available
The Tamron 17-28mm F2.8 is an affordable, fast and high quality wide angle lens with a somewhat limited zoom range. The lens is 99mm long and takes a 67mm filter thread. I used an ND1000 Hoya Pro filter with a thickness of 5.6mm, the lens external radius is 69mm with the filter on.
The lens nominal field of view is 103.70 degrees however all mirrorless lenses have software corrections. Ideally I need to know the real field of view however the error is normally 1 to 3% and does not influence the calculations too much.
If we consider a length of the lens and filter of 104.6mm and a radius of 34.5 mm for an angle of 51.85 degrees we obtain a distance from the lens mount of 77.5 mm and from the focal plane of 95.5 mm. Taking into account that the MOD is 190mm this gives a minimum radius of 94.5 mm for the dome which is very good news.
Sony has recently released the 16-35mm F2.8 GM2 with a MOD of just 22 cm. Using the same logic as before we calculate the entrance pupil to be 93.5 mm from the lens mount or 111.5 mm from the sensor. This means a dome radius of minimum 220-111.5=108.5 mm is required to contain the area not in focus inside the dome.
Dome Selection Part I – Field of View
The first thing that we need to ensure is that the dome field of view can contain the lens field of view otherwise our main objective of preserving the air performance would be lost.
Unfortunately the specifications of dome ports on the market are somewhat lacking so you need to make do with what you have or ask for CAD details.
Here you can see that, ignoring the thickness of the glass for simplification purposes the various ports have the following field of view using the formula 2*arcsin(glass port diameter/curvature radius)
Part Number
Description
Angle of View (degrees)
Widest Lens mm(Full Frame Eq)
18809
180mm Optical Glass Wide Angle Port
109.8
16
18812
230mm Optical Glass Wide Angle Port II
146.8
7
18813/18815
250mm Optical Glass Wide Angle Port II
102.75
18
It is somewhat surprising to see that the larger port in terms of size is the narrower in terms of field of view I believe this is a compromise in terms of weight.
Dome Selection Part II – Curvature Radius
Looking at field of view is not sufficient, we also want to ensure that the lens MOD is contained by the dome and therefore we need to take into account the actual radius of curvature of the port
18809 180mm Optical Glass Wide Angle Port Radius 110mm
18812 230mm Optical Glass Wide Angle Port II Radius 120mm
18813/18815 250mm Optical Glass Wide angle port Radius 160mm
The port size goes with the curvature radius however somewhat surprisingly the difference between the 230mm and 180mm port is rather small making the choice between the two more a matter of field of view.
Amount of Recession of the Camera from the Port
Camera housings are not like skin, armed with a digital calliper you need to determine the distance between the lens mount and the housing port mount. Alternatively you can reverse engineer this once you have a lens port combination that is absolutely exact.
For the purpose of my calculations I have measured that my E-Mount camera is 27mm recessed inside the housing. This is important as it is needed to calculate the extension for the dome. If you are in a different format you need to measure this distance yourself.
Entrance Pupil to Housing Port
We have previously determined the entrance pupil from the lens mount and now we know how much this is recessed in the housing so we can calculate the required extension to reach the entrance pupil however this assumes the domes are hemispheres which in most cases they are not. Let’s leave this aside for one second and go back to our examples.
Sony 20-70mm F4 Entrance Pupil Distance = 73.79mm – Housing recession 27mm = 46.79mm from the housing
Sony 20mm F1.8 Entrance Pupil Distance = 62.91mmm – Housing recession 27mm = 35.91from the housing.
Tamron 17-28mm F2.8 → 50.5mm from the housing
Sony 16-35 F2.8 GM2 → 66.5mm from the housing
Wide Angle Ports
Again we need a calliper to determine the depth of the port as those are not full hemispheres. I have access to the 180mm dome and I know that the port is actually 8.5cm tall from mount to glass edge because I measured it. This means I need to add 25mm to the extension required. From the manual I estimate the 230mm port needs extra 13mm and the 250mm port 34mm. if you own those ports and want to provide me the exact measurement I will build a calculator for dummies.
Back to our examples with some real calculations:
Sony 20-70mm F4 Port required 250mm Wide angle Port. Extension required 46.79+34mm=80mm
Sony 20mm F1.8 Port Required 180mm Wide angle port. Extension required 35.9mm+25mm=60.9mm
Sony 16-35 F2.8 GM2 Port Required 180mm Wide angle port. Extension required 66.5mm+25mm=91.5mm
If we are using the 35.5mm N100 to N120 adapter this means that the actual extension rings required are
Sony 20-70mm F4 → 80-35.5=45 mm Part required N120 Extension Ring 45
Sony 20mm F1.8 → 60.9-35.5=25.4 mm Part Required N120 Extension Ring 25
Tamron 17-28mm F2.8 → 75.5-35.5=40 mm Part Required N120 Extension Ring 40
Sony 16-35mm F2.8 GM2 → 91.5-35.5=55 mm Part Required N120 Extension Ring 55
Nauticam Port Chart Check
Lens
Extension Determined
Extension Suggested
Delta
Sony 20-70mm
45 mm
40 mm
5 mm
Sony 20mm
25
NA
NA
Tamron 17-28
40 mm
40 mm
0 mm
Sony 16-35GM2
55 mm
50 mm
5 mm
We can see that for the 20-70mm where the entrance pupil is known the Nauticam port chart is off 5mm. For the 17-28mm where design information is not known there is no discrepancy with my method and for the 16-35 GM2 there is a discrepancy of 5mm.
I checked the situation for the 16-35mm GM2. This lens has some distortion and therefore the uncorrected field of view is 109 degrees which is too big for the 180mm wide angle port. With my calculated 55mm if you remove distortion correction in camera you can see a tiny bit of shading from the dome petals but this goes away when distortion correction is active. Therefore I am satisfied that my calculations are more accurate. I contacted Nauticam who ran the MFT charts in their test rig and they said 55mm works well too. Although it is ideal to have the exact extension if you have one that is 5mm off the calculated value and the dome does not vignette you need to consider if the image quality you get is satisfactory and make your call.
I also tested this lens in a pool, you can read the review in this post.
What about other brands?
The challenge with other brands is the lack of documentation however you can contact the design department to obtain information on the dome port they should not be a secret. The other challenge is the availability of extension rings. The Nauticam system has a level of precision of 5mm which is excellent however I am under the definite impression that they run their tests using in most cases steps of 10mm and using as first approach how the lens fits the port, they do not go and attempt to determine the entrance pupil.You can observe that because when you look at a specific port say the 180mm wide angle and you apply the suggested extensions in all cases the lens edge is flush with the extension.
In most cases this turns out to be accurate however there are some cases where wider lenses need to be more recessed and narrower lenses need to stick out more.
Fisheye Lenses
When you use a fisheye lens with a complete hemisphere dome port the calculations remain the same however it is a bit simpler to proceed without data. If your fisheye has a diagonal 180 degrees view and your extension is too long you will see vignette in the corners. However if you push your fisheye lens closer to the glass you may be able to use a dome with a smaller field of view but the edge distortion will increase and so will chromatic aberrations. A classic example is the 230mm wide angle port used with a fisheye lens. The port has a field of view of 146.8 degrees which is far away from the required 174 degrees of a diagonal fisheye lens.
Panotools provide entrance pupil for the Nikkor 8-15mm therefore we don’t need to go trial and error. The Canon 8-15mm Fisheye is on the Optical Bench Hub.
Following the same logic we determine that the entrance pupil is 129.98-18-17.98-27=67mm from the housing. Taking out 35.5mm for the adapter we get 31.5mm vs the 30mm on the Nauticam port chart. This means the lens will stick out from the dome opening and that is fine as a shorter lens would make it vignette. Try it if you have a 35mm extension you will see the vignette. If you have access to all extensions in steps of 5mm you can determine the correct one when the vignetting stops even without the entrance pupil position. Please note the above calculation is to use a Canon EF lens (flange distance 44mm) on E-Mount (flange distance 18mm) however if you work that out in the N120 Canon system with the additional gap for the more recessed housing you end up in exactly the same place.
Wrap Up
This article has shown that it is possible, with basic knowledge of trigonometry and access to lens, dome and camera design information to determine:
How well a lens may work
What is the minimum dome radius required to preserve the image quality
What is the extension required
How to find out the required field of view of your port
Without acquiring the actual lens camera or wide angle port. It is important to understand that if a lens is weak in air it won’t get better in water and in particular you need to appreciate that topside tests are not identical to use behind a dome that instead means working at very close focus well under one metre mostly around 30 to 50 cm. It may be worth it in some cases to rent a lens if available and take some tripod shots at close range. If you see really weak performance the lens may not be worth housing it at all.
One of the misconceptions that has propagated in the last few years is that all rectilinear lenses offer poor performance compared to water contact optics and you need to stop down to very small apertures to have good edges. From my personal experience with the Tamron 17-28mm, I can conclude that this lens is far sharper than the water contact optics that require a similar investment and even beats more expensive options (WACP-C) however the rectilinear look is somewhat not in trend in underwater photography. Majority of photos want the centre to pop and this works better using fisheye lenses or distorted optics at close range. Still there is a place for rectilinear lenses: models, wrecks where shapes are known, even fish and marine life where the exact dimensions matter for scientific purposes.
I hope that this article allows you to have a more informed view of the key factors to look for in a lens that will ensure underwater performance is as good as it can be.
In a previous post I described the use of the Canon 8-15mm as a zoom fisheye using the Kenko Teleplus HDpro teleconverted.
I had the opportunity to try this set up in Malpelo although in a situation that was not ideal for it.
I put this lens on expecting some wide angle school shots and instead it ended up being a dive with Galapagos sharks coming fairly close.
With the imminent launch of the Nauticam Fisheye Conversion Port many users will ask if they shoud invest in that or they can get decent quality at more affordable cost spending less then £800 for a teleconverter set up. I assume any Sony full frame E-mount shooters own both the Canon 8-15mm and the Sony 28-60mm.
Edit 9 March 2024
Studio Shots
I found some time to do some tests at f/16 distance 25cm which is typical of wide angle in a dome.
As you can see the kenko 1.4 TC does not loose any quality compared to the bare lens and looks more magnified at same focal lenght in the centre.
Canon 8-15 15mm f/16Canon 8-15 1.4 15mm f16
Why is the quality the same? Probably the Canon 8-15mm is a better lens at 10.7mm that it is at 15mm and therefore even with the teleconverters result match. This corroborates my in water results.
Malpelo Shots Analysis
The dive was early in the morning and topside overcast resulting in a fairly dark dive.
The sharks came fairly close however as soon as the strobe fired they turned on their back. My impression was that this was more due to the noise of the strobe firing then the actual light.
All my shark shots are at f/8 1/30 ISO 500. As the shutter speed is quite low you have situations where the subject is sharp but some of the fish at the edges has some motion blur this is unrelated to the lens.
Profile
If you open the above image on a separate tab and zoom 100% you will see that the shark is pin sharp and so are the small fish on the same focal plane and the one behind. The reef on the left bottom corner is soft.
This has to be expected as the focal point is further back from the shark so the camera is out of depth of field on that corner.
The situation repeats in other shots like this one where the shark is even closer however the edge improves due to the reduced distance gap with the reef.
Checking inTurn back
Again shot after shot the fact I was focussing on the shark that was deeper in the frame resulted in the left edge being soft, this has to be expected and there is nothing wrong with the set up the dome or else.
I took some shots really close on the reef at f/16 to make the point here the muray eel is sticking out of the reef so the edges look much better.
This other shot has an hawkfish in the edge you can still see the coloration and the eyeball of the fish.
Conclusion
In general terms shooting f/8 on full frame is not an example of small aperture in fact this is a setting for distant subjects almost and wide angle scenes. In the environmental situation I was in I could have increased the ISO to achive higher shutter speed and smaller aperture however this would have resulted in more noise and loss of resolution across the whole frame. My view is that for general wide angle where there is no clear subject you can try to focus closer to have the edges sharper however this is not always a possibility with sharks and things moving and furthermore there is rarely anything of interest in the edges.
This was my second time with this combination and I remain of the opinion that the teleconverter does not take anything away in the center of the frame and deteriorates the edges only just slightly and is therefore a worth addition. The nauticam FCP is not yet released and combined with the 28-60mm will for sure produce a more flexible set up because of the increased zoom range compared to the teleconverter however if this produces better image quality on the overlapping range remains to be seen. I expect it will cost considerably more than the £800 required to add the teleconverter to your Canon 8-15mm.
Few days ago Alex Mustard popped in to drop his WWL-DRY aka WACP-C prototype so that I could conduct some experiments for the enjoyment of the entire underwater community on Sony E-mount.
This lens is not the same of the current WACP-C but it is very similar. It does not have a float collar, a bit like the original WWL-1 dimensionally appears a few mm different from the WACP-C specs.
The lens seems a bit shorter.
140 mm length instead of 145 mm of current production version
The dome diameter is identical somewhere in the region of 130mm.
Dome port perspective masks the real diameter of 130mm
The lens is very heavy in water so I needed some floatation.
Stix float belt carved to fit a dome
I rented a Tamron 28-75mm G2 from lenspimp only to discover it would not fit any of my extensions. Alex Tattersal has sent me an adapter on loan but it did not make it for my pool session.
I therefore decided to use my Tamron 17-28mm although the extension was 5mm too long I got no vignetting at 26mm.
Ready to dive
I exchanged notes with Alex who told me he tried all sorts of optics with his Nikon only to use a 1990 lens now discotinued as all modern fast lenses would refuse to work properly. I was determined to try anyway confident I would get good results.
Pool Tests
Arrived in Luton for a short one hour session last night I took my usual props. The first set of tests show already some interesting results.
I always start as close I can get to the props to fill the frame.
CFWA f/5.6 T28
At f/5.6 the centre is very sharp however I noted the background and were not particularly crisp while the centre was but not in the background. There is an issue of depth of field so I started stopping down the lens.
CFWA f/8 T28
By f/8 results were already very good considering the shooting distance. Consider that a shot like tha requires f/16 on a fisheye or rectilinear to have sufficient depth of field.
By f/11 we are in a really good place.
CFWA f/11 T28
The depth of field is not quite enough for the plant in the back but the edges are sharp.
To show that this is a genuine depth of field issue look at this shot at 17mm in APSC.
17mm APSC f/5.6
It looks very much identical although this is even wider at 25.5mm equivament.
The second step is to look at edge sharpness the pool provides a nice tiled wall for this purpose. Here am shooting at around 1.5 meters.
You can see immediately that the frame is sharp throughout at f/5.6
wall f/5.6
Moving to f/8 improves edges
wall f/8
f/11 brings better edges but in my opinion not the best centre.
wall f/11
This reflects very much the nature of the master lens which is outstanding in the centre at f/5.6 with so so edges but very good on both accounts at f/8. F/11 starts showing an overall resolution loss.
I then moved to test field of curvature.
grid f/5.6
The lens has virtually no field of curvature and the edges are good already at f/5.6.
grid f/8
By f/8 the result is excellent.
grid f/11
At f/11 better edges but slightly worse centre.
Having completed the lab tests it was time to shoot some divers however I was coming to the end of the hour and they had started surfacing!
group f/5.6
Shots at distance with f/5.6 look great.
surface 3 f/5.6
Consider the shutter speed is low as I was trying to get some ambient light and the subject far so there is some motion blur.
surface f/5.6
f/8 is probably the sweet spot for underwater use.
Wide f/8 T28group f/8
F/11 is really not needed unless you have a close up shot.
Self Potrait f/11 T28
Conclusion
There are some obvious strengths to the Tamron 17-28mm which in my view performs at 28mm way better than the Sony 28-60mm even with a too long extension.
Upon reflection I have decided not to invest on the Tamron 28-75mm as I already have thr Sony 24-70mm GM2 and there is an overlap topside.
Edit 8 April: I received today the adapter ring I needed for the 28-75mm G2 and unfortunately there is vignette at 28mm ruling this lens out entirely for the WACP-C.
If you want to use the Tamron 17-28mm with the WACP-C you need an N120 to N100 25mm adaptor ring, in addition to the zoom gear (not necessarily unless you want to shoot also APSC) and the 35.5mm N100 to N120 port adapter.
The Tamron 17-28mm costs $799 on Amazon.com and it is the best rectilinear wide angle for underwater for the e-mount and we now discovered also compatible with WACP-C.
I will try other lenses in due course but the lesson learnt is that if you do your homework you will find something.
Thanks to Alex for the loan and bear with me a little longer!
The subject of rectilinear wide angle lenses and underwater optical performance has been beaten to death.
Most recently some photographers and videographers have done without rectilinear lenses altogether citing the horrible performance at the edge of the frame as the primary reason to shoot lenses with barrel distortion being those fisheye or standard lenses with an added water contact optic.
Is it all justified? Should you stay away from rectilinear lenses altogether?
Of course not. Rectilinear lenses have a place in underwater imaging that is there to stay but…
There are many many buts so let’s dive into some demistification and general considerations.
Topside Performance of Wide angle lenses 17mm use case
The vast majority of underwater shooters do not perform any type of topside imaging being that photo or video and use their set up only underwater with the exception of the odd event or those that like to shoot macro above and below this is where we stand in most cases.
In order to ascertain if the performance of the wide angle lens in water deteriorates it is appropriate to determine the performance of said lens topside. This is something that not many people are in fact able to ascertain. You can read equipment review but it is never the same thing.
I have recently invested in a Sony A1 and underwater housing and I decided to purchase a rectilinear wide angle lens. I always try to buy lenses that are good topside and underwater and my choice has fallen on the Tamron 17-28mm F/2.8 Di III RXD.
Several reasons for buying this lens I will list the most important are:
Close minimum focus distance (19 cm wide – 26 cm tele)
Lens does not extend when zooming
Reasonably compact (99 mm and 420 grams)
Not too wide
Items 1,2 above are important underwater and 3,4 are also important topside when you use the lens on a gimbal or when you shoot interiors or close up and you do not want apparent perspective distortion.
I have been shooting the Tamron topside on trips and I have been very pleased with it. Before taking it underwater I wanted to understand what to expect.
I built my scene using my underwater props on a table top and started taking a series of shots from f/8 to f/22 to see the results. What follows are a set of images taken on land with the focus on the eye of the chick.
Topside f/8 subject
At f/8 the first row of props is blurry and also the leaves behind are not sharp. The edges are soft.
This is due to lack of depth of field not to the bad performance of the lens.
Topside f/11 subject
At f/11 we have more depth of field however the props at the edges and the nearest part is still soft.
Topside f/16 subject
By f/16 all the props are in focus, the edges of the math are a tad soft but overall this is the right place to be.
Topside f/22 subject
By f/22 pretty much everything is in focus but the image quality has dropped considerably.
In conclusion topside we need to close down to f/16 to have all the props in focus on this scene.
Just so we are clear there is not an issue of edges or else there is simply lack of depth of field as we are shooting a close up.
Underwater Performance of Wide angle lenses 17mm use case
After rigging my camera and lens with the Nauticam 180mm dome it was time to hit the pool and try underwater.
What follows are a series of shots from f/8 to f/16 with focus on the chick as the topside shot.
Tamron 17mm f/8 Focus Mid
The image at f/8 looks very much like topside and lacks depth of field, however interestingly the bush behind the chick is relatively sharper to the topside scene. The dome port is increasing the depth of field behind the subject.
Tamron 17mm f/11 Focus Mid
By f/11 there is a considerable improvement all across the frame the items at the edges are still soft pretty much like the topside image.
Tamron 17mm f/16 FocusMid
By f/16 we are where we should be. Note how the entire set of props is in focus exactly like the topside image and only a slight deterioration on the very left of the frame prop.
Move your focus…
Dome ports increase the depth of field of the lens as infinity becomes nearer however they also shift the depth of field of the lens because the virtual image is closer and most of the depth of field shifts behind.
With that in mind I focussed on the first prop to see what happens.
Tamron 17mm f/8 Focus Near
At f/8 the first line is sharper even the props at the edges are substantially better and I would say acceptable however the leaves behind are out of focus.
Tamron 17mm f/11 Focus Near
At f/11 the edges are good and so are the props behind the chick with the exception of those really further back where we are running out of depth of field.
Tamron 17mm f/16 FocusNear
By f/16 focussing on the near prop we have pretty much everything in focus.
As final example this is a shot at f/11 with focus just behind the fake coral red and yellow on the right.
Tamron 17mm f/8 FocusSide
With the exception of the very very edge of the right frame which is bordering the dome edge the image is sharp throughout.
Why are the images not blurry?!?
You are now starting to ask why some images you see on the web look horrible and mine don’t? This is a legitimate question to which there are at least five answers.
Lens working distance
The Tamron 17-18mm has a working distance of 19 cm and is 9.9 cm long. From my calculation the entrance pupil is around 26mm from the front of the lens. This means that a dome radius of 10cm is sufficient to be able to focus right on the port. All other lenses for the Sony E-mount have working distance of 25 to 28 cm and would need extremely large radius to be able to focus on the glass.
Wrong Extension Ring
The Nauticam 18809 wide angle dome port is not a classic dome but has design without a flat base. The port has 11cm radius of curvature and is 83mm deep 180mm wide this means the entrance pupil needs to be 27mm behind the extension.
Nauticam 18809 180mm Wide angle dome port
Nauticam recommended extension is 40mm when combined with the 35.5mm N120 to N120 port adapter. Due to the shape of the lens this cannot be used with the N100 port as the zoom is close to the front of the lens.
Tamron 17-28mm with Nauticam zoom gear
Underwater housing manufactures unfortunately do not apply any science to the selection of domes and extension for a lens but out of pure coincidence the 40mm extension ring is what this lens requires.
Tamron 17-28mm 35.5 adapter and 40mm extension
Nauticam criteria for the 180mm port seems to be the lens needs to be more or less flush with the extension ring. In our case the entrance pupil is 26mm behind and the extension ring is a few mm over so we are more or less spot on. THIS IS ENTIRELY A COINCIDENCE!
With the 40mm extension the glass port will be exactly 11 cm from the entrance pupil and focus right on the surface.
Unfortunately the principles applied by manufacturers which are either to be flush with the ring or the dome mount or to extend until it vignettes generate loss of angle of view and distortion.
One of the worst offender is the Nauticam 230mm glass dome, where the rule of extend until you can generates pincushion distortion (edge pulling) effect.
Software Correction of Distortion
The lens correction has a warping effect on the edges even when the dome is correctly placed.
Exteme edge at f/11 with distortion correction offExtreme edge at f/11 with distortion correction on
If you have a full frame camera or a camera where you can disable the correction make sure this is set that way.
For systems where lens correction is baked in the lens profile (micro four thirds) ensure to use a program that can disable the correction such as DxO Photolab for best results.
Constant Autofocus with subject tracking
As we have seen in most cases it is better not to focus bang on the subject but to focus closer or even at the side of the frame and work out the depth of field.
Many users use subject tracking that may seem a wonderful idea and works very well with a flat port however underwater results in blurred edges due to the depth of field distribution of the dome port unless you close the aperture until you can care less.
If you want to ensure the edges of your rectilinear wide angle shots are sharper use single autofocus and position the focus strategically in the frame the dome compression will do the rest.
Superwide lenses (<16mm)
When your lens is superwide you may have all effects on top of each other: pincushion distortion because your extension is calculated with the ‘go until vignette’ method, distortion correction in software, subject tracking, lens with long working distance and to make it worse perspective distortion and spherical aberration which occur when the lens is very very wide.
Choosing your rectilinear wide angle lens
I have computed all lenses avaialble for my A1 and calculated the ideal radius of a dome in this table. Unfortunately while the 230 port has a wider angle of view it has a radius of 12 cm which is only 1 cm more than the 180mm port. The choice of port is therefore driven by angle of view and not radius as 1 cm does not change much. Most lenses would need 15 or 16 cm radius to be able to focus close to the glass.
Brand
Model
Working Distance
Field of View
Radius Required
Port
Sony
SEL1224GM
280
122
152
230
Sony
SEL1224G
280
122
152
230
Sony
FE14 1.8 GM
250
114
157
230
Sony
SEL1635GM
280
107
169
180
Sony
SEL1635Z
280
107
169
180
Sony
PZ1635G
280
107
169
180
Sigma
1424DGDN
280
114
164
230
Tamron
1728RXDIII
190
104
99
180
Zeiss
18mm
250
100
177
250
Sony e-mount rectilinear lenses and ports
You can easily see that the Tamron 17-28mm has much better design characteristics to go into a dome port and this is the lens to buy.
From my tests there is no need of any field flatteners and correction lenses it works fine out of the box as the lens has minimal field of curvature and zero spherical aberrations.
Pool Session
If you are not happy with the CFWA studio scene here are some images from a recent pool session. Many f/8 and f/11 images no need to close the aperture more if you don’t have anything close and shot in single autofocus.
Skill training 17/8Dave Side 17/8Female Diver Side 17/8Drysuit Diver Front 17/11Maddy Portrait 17/11Rush Diver Side 17/11Manu Side 17/11Dad Daughter 17/8
Quite interesting to see the fins at the edges of the f/8 shots.
If there is one challenge of rectilinear lenses is that you need strobe power. The narrower field of view compared to a fisheye means that you are standing further back that in turn means more strobe power and more particles between you and the subject.
Should I buy a rectilinear wide angle lens for underwater use?
My answer is definitely yes but each system will have the special lens, the one that focuses close, does not require a large radius and is not too wide and not too narrow (16-18 is ideal) however examine carefully images of others and check the lens construction as your suggested extension may be wrong.
But when your lens is fit for purpose using the correct size dome and the proper extension you will get high quality images that match or beat fisheye like lenses and water contact optics.
As example here an image shot with a canon 8-15mm at close range f/11. Does it have much better edges?
VideoDiver
And here a WWL-1 image at f/8
WWL-1 f/8
I cannot see any advantages of the fisheye like lenses at the edges at the same aperture in fact in both cases but judge for yourself.
Unfortunately Phil used the Sea and Sea correction lens an expensive accessory that this lens does not require but as soon as I am in open water I will post images myself.
Since many years Canon and Nikon full frame users are able to use their respective 8-15mm with a teleconverter underwater, however this is not a very popular configuration.
In this article I will look at the Canon 8-15mm with the Kenko Teleconverter 1.4x for Sony full frame cameras.
First and foremost a teleconverter is not cropping the image it has optical elements. Cropping means reducing the resolution at sensor level while a teleconverter induces a deterioration of the image and possible defect but does not affect the sensor resolution. Generally 1.4x TC is much better than 1.4 crop. If you find yourself cropping a lot your fisheye shots or even using the 8-15mm in APSC mode the teleconverter may add some real value to you so read along.
Parts Required
In addition to the set up required to use the Canon 8-15mm you need 3 additional items:
Kenko Teleplus HD Pro 1.4 DGX
Kenko 1.4 Teleconverter
Canon 8-15+TC zoom gear
Extension ring N120 20mm
N120 Extension ring 20Canon 8-15mm with Tc and gear
The benefits of this set up are clear:
Unique field of view
Smaller additional bulk
Relatively low cost
Some readers have emailed asking if the Kenko is compatible with the Sigma MC-11. I do not recommend using the Sigma MC-11 with the Canon 8-15mm because it only supports single AF and it is unclear if the Kenko will work or not and how well. I have tested with the Metabones smart adapter and this is the one I recommend.
Field of view
The 8-15mm lens with teleconverter will give you access to a zoom fisheye 15-21mm with field of view between 175 and 124 degrees. This is a range not available with any other lens of water contact optic that stop normally at 130 or 140 degrees.
Additional Bulk
The additional items add circa 370 grams to the rig without teleconverter and make is 20mm longer due to the additional extension. The additional fresh water weight is circa 110 grams.
Cost
The latest version of the Kenko Teleplus 1.4X HD DGX can be found in UK for £149.
The 20mm extension ring II is £297 and the C815-Z+1.4 Zoom gear is £218. Note this is in addition to the 30mm extension required for the 8-15.
With a total cost of £664 you are able to obtain the entire set up.
The rig looks identical to the fisheye except is a bit longer. You have a choice of 140mm glass dome or 4.33″ acrylic dome see previous article.
Additional extension ring on otherwise identical rig
With the rig assembled I made my way to the pool with the local diving club.
Pool Session
The 8-15mm with teleconverter was my first pool session with the A1 on the 3rd of February I was very much looking forward to this but at the same time I had not practiced with the A1 underwater previously and did not have my new test props. I think the images that follow will give a good idea anyway.
15mm Tests
At 15mm (zoom position somewhere between 10 and 11 mm on the lens) the image is excellent quality in the centre and I find very difficult to tell this apart from the lens without TC except for the color rendering. I believe the Kenko takes a bit away from the Canon original color rendering.
Peter at 15mm f/11Dad and Son 15mm f/8Diver girl f/11
At close range you get the usual depth of field issues depending on where you focus but this is not a teleconverter issue.
CFWA 15mm f/8Peter and croc
For comparison a 15mm image without TC.
VideoDiver
Zooming In
Obviously what is interesting it that you can zoom in here a set of shots at 16, 18, 21 mm.
16mm f/818mm f/821mm f/8
Finishing up with the required selfie.
21mm f/8
Conclusion
I enjoyed the teleconverter with the Canon 8-15mm and in my opinion in the overlapping focal length this set up provides better image quality of the WWL-1. I shot for most at f/8 as I was not very close and this actually shows the TC does not really degrade the image much.
You need to ask yourself when you will need 124 to 175 degrees diagonal and the answer is close up shots of mantas and whalesharks where a fisheye may be too much and 130 degrees may be too little. The set up also works if you want to do close up work and zoom in however I reserve the right to assess more in detail using my new in water props when I have some time.
Since the very first release I was told by Nauticam that the WWL-1 had been tested on Sony full frame with the 28mm f/2 lens and since then more lenses have been added to the compatibility list and the WWL-1 itself has had a redesign called WWL-1B, this lens has an integrated float collar and I do not know if there is any difference in the optics but I assume there is none.
Nauticam WWL-1B
Nauticam has since released a number of other water contact optics with dry mount and today you have a choice of at least 3 flavours for your Sony full frame camera that provide the 130 degrees diagonal field of view.
Model
Price (€)
Weight (kg)
Diameter (mm)
Depth (mm)
Max Filter size (mm)
WWL-1
1424
1.35
156
97
52
WACP-C
2930
2.24
170
145.5
72
WACP-1
4604
3.90
194
176
82
Summary Table Nauticam 0.36x Water Contact Optics
The three lenses provide the same field of view but they are different in size and mount. A useful way to see is that as the lens physical size grows you require a larger underwater optic.
The Sony E-Mount is still the only full frame format compatible with the WWL-1 in virtue of some really small and compact lenses. As you can see from the table above the WWL-1 rear element is large enough for 28mm lenses that have a maximum filter size of 52mm.
Two E-mount full frame lenses the 28/2mm prime and the 28-60mm zoom are compatible with the WWL-1.
As you move towards the WACP-C you can also use the 28-70mm lens which is one of the worst kit lenses on the market but will give you a longer tele end and finally the WACP-1 gives access to the Tamron 28-75mm and Sigma 24-70mm two lenses that have much higher quality than the smaller Sony lenses but have some restriction in terms of zoom range.
Underwater Performance Context
There are quite long discussions about which water contact optic to get for your Sony full frame once you have the 28-60mm zoom and some comparison in terms of sharpness.
In simple terms you can think of the following equation:
Underwater Performance = Land performance X Port Factor
Port Factor is always less than 1 which means a lens will never do in water as well than it does on land. Looking at my analysis of the 28-60mm corroborated by other test you know before buying any water contact lens that the lens has its own limitations and no matter how good is the port performance will only go down. However this may still be a better option compared to a standard dome port.
I do not have access (yet) to the other two water contact optics however I have a good idea of how the WWL-1 perform and how the Sony 28-60mm performs topside. If you want a refresh look at this article.
To understand how a water contact optic works you can go back all the way to the Inon UWL-100.
The idea of this lens designed for compact cameras is to demagnify the camera master lens to enlarge the field of view. You could then get an optional dome that will enable the lens to expand the underwater field of view from 100 to 131 degrees.
Back in 2015 I compared the Inon UWL-H100 with dome with the WWL-1 and concluded that the WWL-1 was giving better results when used on the same camera. It is now time to see if the WWL-1 can be used also on a full frame system.
Sony A1 WWL-1 Rig
The WWL-1 requires the flat port 45 to be used on a Sony full frame underwater housing. The lens will be attached using the same bayonet adapter that has been available for several years now.
Nauticam Bayonet Mount for WWL-1
I have removed the focus knob from the port as I found it inconvenient. The focus knob may be useful with the flat port but for the WWL-1 that is afocal is definitely not required.
WWL-1 topside view
Once you add the flat port the overall length is very much the same of the WACP-C but this will require an extension ring resulting in overall 30mm additional length.
Overall the rig is very similar in weight to the Canon 8-15mm with the Acrylic Dome Port 5.5″.
WWL-1 front side view
Pool Tests
With the rig assembled I went for a pool session with the objective of finding out what was the overall performance of the system.
What follows are a series of test shots of divers.
David f/8Kid f/8f/11 side shot
WWL-1 selfie 28mmDiver f/10
In general I found the lens to be sharper in the centre at f/8 but closing down to f/11 was required if there was something in the corners.
Edge Sharpness
I was intrigued by a number of discussions on edge sharpness and after several exchanges with Shane Smith he was clear that the lens needs to be stopped down to f/11 for best results.
After the session in the pool I would agree with Shane however I was curious if this was an issue of the WWL-1 or the 28-60mm lens itself.
This image quite simple has something at the edges and has focus in the centre at f/8.
Closeup f/8
You will notice that the part of the frame closer to the camera is fairly blurry.
So I did another experiment placing the slate on the edge.
Fuzzy f/8
The edges were quite fuzzy. I wanted to exclude this was an issue of depth of field so I focussed right on the corner.
f/8 focus on corner
This is the resulting image and is still soft on the edge.
fuzzy f8 focus
I then took the same shot at f/11 with focus on centre.
Closeup f/11
The image at the edges is better. Then moved the slate to the edge.
edge f/11
The image improved overall regardless of the focus point indicating this is not a depth of field issue but some other defect of the lens, most likely as the lens meridional and sagittal resolution are different we have an example of astigmatism.
Edge 100% crop f/11Blurry f/8 edge focussed on edge
The sharpness improves closing down the lens regardless of where the lens is focussing consistent to the MTF charts.
Looking back at land test shots we can see something very similar.
Edge at f/11 topsideEdge at f/8 top side
In conclusion it is not about the WWL-1 but about the lens itself.
Comparison to Rectilinear lenses
While the WWL-1 can offer a diagonal feld of view of 130 degrees the image is distorted and the lens can only offer 107 degrees horizontally and 70 vertically. Is like saying that the horizontal field of view is similar to a 13mm rectilinear lens while the vertical is is more like 17mm. A fair comparison is probably a 14mm rectilinear lens but as the WWL-1 is a fisheye like optic a direct comparison is not entirely possible. In my opinion as the image is distorted is more appropriate to compare the WWL-1 with a fisheye with teleconverter and when I look at what the canon 8-15mm with kenko 1.4 tc can produce for me the results are very similar, I would say the Canon has in fact an edge however the field of view are not comparable except when the WWL-1 is at the widest and the canon with the tc at the maximum zoom. I would go as far as to say that the canon + TC at f/8 is as good as the WWL-1 at f/11.
Canon 8-15mm with TC at 21mm f/8
Conclusion
If you have the WWL-1 from your previous rig it makes absolutely sense to get the Sony 28-60mm and flat port. This combination will give you decent (but not sensational) shots and work very well for 4K video at reduced resolution. I do not believe that this lens can resolve the full 50 or 60 megapixels of the A1 or A7R4 or A7R5 even topside.
If you are starting from scratch I would recommend to think careful at your intended use case. If you want angles wider than 130 degrees and already have the Canon 8-15mm you may want to check the kenko telecovenverter before you buy a new port as all you need is a 20mm extension ring and a zoom gear.
If you really like the field of view range of 69-130 degrees you need to consider which water contact optic you need.
I am still looking for a test WACP-C but until then my general guidance would be to consider simply if you prefer a dry or wet mount.
A dry mount has the benefit of being ready to go as you hit the water, without the need to remove bubbles between the wet lens and the port. As photographer a dry mount may be the best way forward.
If you intend to use your camera for video and insert filters between the lens and the flat port or you require the lens to be removed in water then go for the WWL-1.
Rigorous comparisons between WACP-C and WWL-1 are not yet available but the first indications are that the difference in image quality is very small therefore I would not loose my sleep there and look more at overall ergonomics.
The final consideration is should you get the WACP-1 instead? Based on my assessment of the Sony 28-60mm I would think this is not particularly wise even if this choice is very popular. Personally I always believe that the master lens needs to be good enough to justify the cost of the water optic so I would like to see how the Tamron 28-75mm performs however no test images are available so I am not in a position to conclude.
In my case having seen what the Sony 28-60mm lens can do I am not planning to invest in a WACP-C but I would be very interested in testing one.
The WWL-1 gets my approval also on full frame but it is not going to give me the same resolution than the Canon 8-15mm or the Sony 90mm macro will give. I look forward to testing some rectilinear lenses to see how those compare and this will happen in a week from now so stay tuned.
Costs to get one for your Sony full frame excluding lens:
WWL-1B €1,424
Bayonet adapter €102
N100 45 flat port €494
Total €2,020 vs WACP-C + N100 Extension Ring 30 €3,333
Following from a previous article about not increasing bulk I have considered a few options for the Canon 8-15mm fisheye.
The 8-15mm is not a small lens and due to the different flange distance between Canon EF mount for DSLR (44mm) and Sony E-Mount (18mm) we have a chunky 35.5mm N100 to N120 adapter port that makes the whole set up not that compact.
Dome Options 140mm vs 4.33″
The Nauticam port chart recommends the 140mm glass fisheye dome for the 8.15mm, this port is 69mm radius and is made with anti reflective optical glass and weights 630 grams.
140mm Glass Dome on Scale
There is another dome from Nauticam the 4.33″ acrylic but this does not feature on the port chart for the Canon 8-15mm.
I did some calculations and this dome should require the same extension so I ordered one conscious that this would be lighter but not necessarily increase the underwater lift due to a reduced volume.
4.33″ dome weight
Although there is a difference of 362 grams the smaller volume will result in less buoyancy 348g lift vs 688g lift for the 140mm so overall the additional buoyancy is only 22 grams.
4.33″ vs 140mm
The primary benefit of this smaller dome is that it gets you closer this in turn means that things will look bigger and as consequence depth of field will drop. Depth of field depends on magnification and as you will get closer it will drop compared to other domes. So larger domes have more depth of field not because they are larger when you are at close range but simply because your camera focal plane is standing further back.
To give an idea this is a little miniature shot with the 140mm dome with the target touching the glass port.
140mm dome close up
This is the same target with the 4.33″ dome.
4.33 dome
Side by side shows the difference in magnification.
Left 4.33″ dome right 140mm dome
If we look at the same detail we can see that the 140mm dome image detail is less blurred.
4’33 dome vs 140mm dome
We are on land here there is no water involved and the 140mm image is sharper at the edge simply because it is smaller.
As depth of field must be compared at equal magnification we can also bust another myth of larger domes vs smaller domes there is no increased depth of field you are just standing further back if you compared the front of the port instead of the focal plane.
Building the Rig
The extension required is still 30mm as for the 140mm dome,
Acrylic dome profile
The overall size of this dome means it is flush with the extension ring.
Port details the lens hood must be removed
This is the overall rig with the amount of flotation in this image it is around 600 grams negative in fresh water.
4.33 rig
Now that we know what to expect is time to get in the pool and take some shots. I got some miniature aquarium fixtures to simulate a close focus wide angle situation.
Pool Session
Once in water I set up my artificial reef and got shooting.
I was at the point of touching the props so I had to stand back a little. As expected the issue is depth of field.
Shots at f/11
For starter we try to get as close as possible and focus in line with the chick.
Fisheye f/11 Focus on back
Due to the extreme magnification the front details are quite soft. So from here I start moving backwards a little.
Still focussed on the chick the sharpness improves due to reduced magnification this is a simulation of a larger dome.
Fisheye f/11 Focus on chick
There still is severe blurring of the front detail at f/11. However due to the increased depth of field that the dome brings behind the focus point the rest looks pretty good.
Focussing on the middle of the frame at f/11 results in blurry details for the features in the front of the frame but much less blurry than before and the chick is still relatively sharp.
fisheye f/11 Focus on edge front
Focussing on the pink reef detail results in a better overall result in a counterintuitive way.
Shots at f/16
Stopping down the lens results in increased depth of field so more of the image is in focus however the overall sharpness drops. This is a good place to be if you don’t want to be too sophisticated with the choice of focus point and you are close.
You can get closer but the front detail is still a bit soft but acceptable.
Fisheye f/16 Focus on back
If you move your focus point a bit further in front the situation improves.
fisheye f/16 Focus on middle
At this point I decided to get into the picture with a white balance slate.
Fisheye f/16 Focus on back diver
Although the front is quite blurry due to the extreme close range the result is acceptable for the non pixel peeper.
Shots at f/22
We are here hitting diffraction limit and the image looses sharpness but we are after depth of field so be it.
fisheye f/22 Focus on duck
Now the depth of field is there although the detail in the centre is less sharp.
fisheye f/22 Focus on middle
Moving the focus point makes the image a bit better.
Time to insert the diver in the frame.
Fisheye f/22 Focus on back diver
Overall ok not amazing consider the dome is on the parts.
Conclusion
The small acrylic dome does quite well at close range, the limitations come from the depth of field and not from the water and the dome increases the depth of field behind the focus point. This is something that you can use to your advantage if you remember when you are in open water.
For shots that are further away you can shoot at f/11 and get excellent IQ there is no need to stop down further to improve the edges. Consider however that f/8 may be just too wide on full frame and introduce additional aberrations regardless of depth of field.
VideoDiver at f/11
Some numbers:
Nauticam 140mm Glass dome: £911
Nauticam 4.33″ Acrylic dome: £550
Price difference £361 or 40% however bear in mind that the primary benefit of the glass dome is to resist reflections and ghosting due to the coating and the fact you can keep the 8-15mm hood on.
When shooting ultra-wide angle, you benefit from a large depth of field
You can get very close to large subjects, maximizing color and sharpness
They perform well behind dome ports with good corner sharpness, and they don’t need a diopter
You usually need at least 2 strobes with good angle of coverage to properly light the entire area.
Some of the above statements are correct in absolute, some are correct but not specific to fisheye lenses and some are just incorrect.
Fisheye lenses usually focus very close -> true for the most recent fisheye lenses, not true for some older models
They are small and light -> Not true. Canon 8-15mm and Nikon 8/15mm are fairly chunky lenses with lots of glass
When shooting ultra-wide angle, you benefit from a large depth of field -> not a property of the fisheye lens but of the focal lens. In fact due to the extreme field of view Fisheye lenses have issues of depth of field.
This is a tea towel shot with a rectilinear lens. Note how sharp the target is at f/5.6
Rectilinear f/5.6
This is the same target at the same distance with the Canon 8-15mm at f/5.6 note how the edges are blurry and the blur starts very near centre.
fisheye f/5.6
You need to stop down the lens to f/16 to start getting coverage for the edges.
fisheye f/16
You can get very close to large subjects, maximizing color and sharpness -> This is a consequence of close working distance and wide field of view however sharpness is another story
As we have seen before fisheye shots at close distance are generally not that sharp especially at the edges.
They perform well behind dome ports with good corner sharpness, and they don’t need a diopterThis happens to be true in practice and it is a major benefit for the underwater shooter
We will dive in detail in this topic.
You usually need at least 2 strobes with good angle of coverage to properly light the entire area.Fisheye lenses cover an aspect ratio wider than the format aspect ratio and result in limited vertical angle of coverage. Fisheye lenses are ideal for two strobes except the very far edges.
A barrel gives an idea of the fisheye lens distortion
Let’s ignore the edges and assume we are a one meter.
Horizontal field of view 2*tan(71)=5.8 meters
Vertical field of view 2*tan(45.5)=2.03
Aspect Ratio = 2.85:1
The issue with fisheye lenses is that the frame is really very wide much wider than it is tall. This means some of the edges on the horizontal axis will be normally dark unless you are very very close.
Fisheye lenses and Dome Ports
A dome is simply a lens with a single element that has the property to retain the air field of view of a lens.
A dome is a lens with a lot of field of curvature simply because it is bent.
Using the dome port visualiser we can see that the effect of a dome is to bring the image closer to where it really is.
The net effect of a dome port is to increase the depth of field as infinity focus is reached much sooner.
A dome port has several side effects the main ones are:
Spherical aberration
Field of curvature
A fisheye lens works opposite to a dome. The centre of the frame is closer to the lens the edges are further away.
Domes, field of curvature and Fisheye lenses
In order to understant how the barrel distortion works in combination with a dome port and a fisheye lens we can build a small simulation in a light box where the edges of the frame are closer than a flat target.
Target in a lightbox focussed head on
We can see that despite the edges are quite blurry this image is actually better than our flat target.
f/11 centre
At f/11 the image is not perfect but we can see that most details off centre are not looking bad at all.
f/11 edgef/11 detail crop
it is definitely blurry but not as bad as the tea towel as if the way the element are laid out improves the image in the corners.
And this is exactly the point: the items as laid out emulating the curvature of a dome improve the fisheye lens performance.
By f/16 the image is almost all sharp.
f/16 centref/16 edgeF/16 Centre 100%
One trick is not to focus in the back of the frame but find a middle point this means we can find additional depth of field in front of the target.
Focus mid way
Let’s see how this goes. at f/11 we already get some better results.
f/11 off centref/11 edge off centre
f/11 off centre crop
At f/16 we get some additional improvement but is not as major as the original f/16
f/16 off centre
Looking at the other areas there are some minor improvements but generally less as we close down the aperture.
f/16 off centref/16 off centre 100% crop detail
In conclusion the layout of the image elements helps the fisheye lens to achieve better image quality this can be futher improve focussing off centre however closing down the aperture results in the best results regardless.
In short we can improve an image at f/11 by shooting off centre in a strategic point to improve depth of field but ultimately aperture plays a bigger role in improving performance of the fisheye lens.
A similar reasoning can be applied to dome size vs closing down the aperture.
We can plot a scenario in the dome simulator tool.
In the starting example our aperture is 4cm to similate our 15mm lens at f/4.
6″ dome f/4 simulator
We now reduce the aperture to 2cm which is more or less f/8
6″ dome f/8 simulation
And finally to 1cm which is more of less f/14. In reality this is mm not cm but should make you understand that aperture matters more than anything else.
6″ dome f/14 smulation
What we can see is that by reducing the aperture the light rays passing through the dome converge and this means stray light is reduced and as consequence spherical aberrations are decreased.
Let’s now introduce dome size which is the equivalent of depth of field in the mix in our light box shooting off centre.
12″ dome f/8 simulation
We can see that with a double size dome the converging effect on the light rays is not as significant as the aperture is already small, but nonetheless is present. This is consistent with our f/11 off centre use case.
Finally at aperture completely closed.
12″ dome f/14 simulation
Although virtual distance has increased significantly the effect of the large dome on the stray rays is not significant here aperture rules.
What does all of the above mean?
I realise this was a bit geeky.
To summarise a dome has two issues one is spherical aberration for the very shape of the dome. This is mostly cured by closing down the aperture. Dome size has limited effect here unless you shoot wide open and with apertures from f/14 we can see that large dome vs small dome does not really matter.
However when it comes to field of curvature large dome helps the situation but because fisheye lens have barrel distortion and this has a counter effect to dome shape curvature therefore dome size matters much less to a fisheye lens than it would to a rectilinear lens.
Some additional insight in this post. And the summary finding here.
The takeaway message is this: stopping down the aperture improves field curvature and astigmatism somewhat, improves coma a lot, and improves spherical aberration most of all. The sum total of these effects changes our ‘area of best focus’, which is what we photographers really mean when we say ‘field curvature’.
We could paraphrase this by saying:
A dome port increases depth of field and a fisheye lens, due to barrel distortion, benefits from a dome port. Optical aberrations introduced by the dome are mostly addressed by stopping down the aperture. The size of the dome port does not matter too much when using a fisheye lens and the benefit on aberrations of a much larger size dome is likely to be minimal when we look at that simulator. Focussing appropriately mitigates residual issues of field of curvature of the dome for the fisheye lens.
Underwater proof of concept
I took my Sony A1 with a Canon 8-15mm first and then with a WWL-1 that behaves very much like a fisheye lens.
Let’s have a look at some images shot with Nauticam 140mm dome.
The two buddies at f/8
The image above sees two buddies in the frame almost flat with their fins going back in the frame however the result is much better than the lightbox example as result of distance and dome port increasing field of view and adding curvature to bring the fins in.
This however does not resolve all issues if you focus near like in this example focussed on the eye of the croc
Focus on the eye at f/8
Here the eye is close resulting in the tail being blurred this is an effect of close distance and lack of depth of field despite the dome.
More interesting the nose is even more blurred as the dome brings that even close and blurs away due to field of curvature as the focus point is behind.
In this other example instead of focussing on the eye the focus goes mid frame so the fins are still in decent shape even if deep in the frame at f/8.
Focus midway
In order to prove the concept even more I took some props underwater.
First let’s have a look a shot at f/8 with the WWL-1.
Close up at f/8
As we can see the image is not too bad even in the close area but it is definitely better at f/11
Close up at f/11
What happens if we position the target off centre?
Contrary to our topside example the situation does not improve by focussing on the edge to further prove the issue here is NOT depth of field.
Focus off centre f/8
Here a detail crop the image is still fuzzy despite then focus is right on the spot. Depth of field is not the issue.
Edge focus at f/8
And finally we close down the aperture to f/11.
Edge at f/11
Crop at 100%
Edge at f/11
So here we can see that the underwater interface provides already for the depth of field but moving the focus at the edges does not have such a good effect.
Why? Because this is likely to do with aberrations of the lens itself as shown in my previous post on the Sony 28-60mm.
The combined 28mm with WWL-1 at f/8 means 20/8-2.5 mm aperture when stopped down to f/11 this becomes small enough to cure aberrations (less than 1cm with reduced field of view is sufficient).
For the same reason ASPC and MFT will be able to shoot at wider aperture not because of depth of field but due to smaller lens aperture.
15mm fisheye at f/14 –> 1.07mm physical aperture
8mm MFT fisheye at f/8 –> 1mm physical aperture
Again it is not the depth of field but the aperture size to cure most aberrations.
Conclusion
All Nauticam port chart recommend the 140mm dome and not larger domes. This is aligned with the theory behind this post that dome size ultimately matters but not as much as stopping down the lens and that fisheye are naturally helped by dome port geometry.
This conclusion also extends to water contact optics which are composed by a fisheye like demagnifier and an integrated dome port.
As long as the rear element of the lens is big enough the increased size of the lens does not result in proportional improvement of performance.
To support the empirical evidence of this article you can read this review of the 140mm dome by Alex Mustard.
By coincidence Alex recommends shooting at f/14 or f/16 which means a physical aperture of 1mm which cures all sorts of aberrations.
Considering that the benefit of a much larger dome may be as small as 1/2 to 2/3 aperture stops you may consider going the opposite way and get a very small dome which will result in additional spherical aberration and will need to be stopped down more when shooting very close.
If you use the Nauticam system there are only two ports that are a full emisphere and therefore able to contain a fisheye lens field of view:
140mm optical glass fisheye port
4.33″ acrylic dome port
I happen to own both those ports and in a future article will compare and contrast the two. I will also revisit the topic of dome ports and rectilinear lenses which is obviously different from fisheye lenses.
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