Tag Archives: fisheye lens

Wide Angle Rectilinear Lenses for Underwater Imaging myths vs reality

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:

  1. Close minimum focus distance (19 cm wide – 26 cm tele)
  2. Lens does not extend when zooming
  3. Reasonably compact (99 mm and 420 grams)
  4. 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 off
Extreme 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.

BrandModelWorking DistanceField of ViewRadius RequiredPort
SonySEL1224GM280122152230
SonySEL1224G 280122152230
SonyFE14 1.8 GM250114157230
SonySEL1635GM280107169180
SonySEL1635Z280107169180
SonyPZ1635G280107169180
Sigma1424DGDN280114164230
Tamron1728RXDIII19010499180
Zeiss18mm250100177250
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 17/8
Dave Side 17/8
Female Diver Side 17/8
Drysuit Diver Front 17/11
Maddy Portrait 17/11
Rush Diver Side 17/11
Manu Side 17/11
Dad 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.

Phil Rudin reviewed the Tamron 17-28mm on uwpmag you can find here some open water images https://www.uwpmag.com/?p=uwp-back-issues&issue=119

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.

Canon 8-15mm with Kenko 1.4 Teleconverter

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 20
Canon 8-15mm with Tc and gear

The benefits of this set up are clear:

  1. Unique field of view
  2. Smaller additional bulk
  3. 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/11
Dad and Son 15mm f/8
Diver 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/8
Peter 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/8
18mm f/8
21mm 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.

Canon 8-15mm with 4.33″ Acrylic Dome for Sony A1

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.

Fisheye Lenses for Underwater myths vs reality

There are several myths surrounding fisheye lenses when it comes to underwater use.

A quick tour to underwater photography guide usually a good source of information.

Myth Busting

Properties of fisheye lens as per UWPG:

  • They usually focus very close
  • They are small and light
  • 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 diopter This 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.

Canon 8-15mm specifications.

Horizontal field of view: 142 degrees

Vertical field of view: 91 degrees

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 edge
f/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 centre
f/16 edge
F/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 centre
f/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 centre
f/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.

Canon 8 – 15 mm Fisheye on the Panasonic GH5 Pool Tests

It was time to get wet and test the Canon 8 – 15 mm fisheye on the GH5 in the pool so I made my way to Luton Aspire with the help of Rec2Tec Bletchley.

I had the change to try a few things first of all to understand the store coverage of the fisheye frame, this is something I had not tested before but I had built a little model.

In purple the ideal rectangle built with the maximum width and height of the fisheye frame

This model ignores the corners the red circle are 90 degrees light beams and the amber is the 120 degrees angle. A strobe does not have a sharp fall off when you use diffusers so this model assumes your strobe can keep within 1 Ev loss around 90 degrees and then drop down to – 4 Ev at 120 degrees. I do not want to dig too deep into this topic anyway this is what I expected and this is the frame.

Shot at 1.5 meters from pool wall

You can see a tiny reflection of the strobes together with a mask falling on the left hand side… In order to test my theory I run this through false colour on my field monitor, at first glance it looks well lit and this is the false colour.

False colour diagram of previous shot

As you can see the strobes drop below 50 at the green colour band and therefore the nominal width of those strobes is probably 100 degrees. In the deep corners you see the drop to 20 % 10% and then 0 %.

Time to take some shots

Divers hovering @ 8 mm

The lens is absolutely pin sharp across the frame, I was shooting at f/5.6 in the 140 mm glass dome.

Happy divers @ 9 mm
BCD removal @ 10 mm
Gliding @ 11 mm
Open Water class @ 12mm
Divers couple @ 13 mm
Hover @ 15 mm

Performance remains stunning across the zoom range. I also tried few shots at f/4

9 mm f/4

There is no reef background but looks pretty good to me.

The pool gives a strong blue cast so the shots are white balanced.

If you want details of the rig and lens mount are in a previous post

https://interceptor121.com/2019/11/02/fisheye-zoom-for-micro-four-thirds/

Panasonic GH5 zoom fisheye rig

Fisheye Zoom for Micro Four Thirds

Looking at Nauticam port chart the only option for a fisheye zoom is to combine the Panasonic PZ 14-42 with a fisheye add on lens. This is a solution that is not that popular due to low optical quality.

So micro four thirds users have been left with a prime fisheye lens from Panasonic or Olympus…until now!

Looking at Nauticam port chart we can see that there is an option to use the Speedbooster Metabones adapter and with this you convert your MFT camera to a 1.42x crop allowing you to use Canon EF-M lenses for cropped sensor including the Tokina 10-17mm fisheye. This is certainly an option and can be combined with a Kenko 1.4x teleconverter giving you a range of 14.2 to 33.8 mm in full frame equivalent or 7.1 to 16.9 mm in MFT terms fisheye zoom of which the usable range is 8 -16.9 mm after removing vignetting.

A further issue is that the Speedbooster gives you another stop of light limiting the aperture to f/16 while this is generally a bonus for land shooting in low light underwater we want to use all apertures all the way to f/22 for sunbursts even if this means diffraction problems.

Wolfgang Shreibmayer started a trend time ago in WetPixel https://wetpixel.com/forums/index.php?/topic/61629-canon-ef-lenses-on-mft-cameras/ to use full frame lenses and in this post I want to do a deep dive on what is for me the most interesting lens option the Canon 8-15mm fisheye.

This lens on full frame can be used for a circular and diagonal fisheye but Wolfgang has devised a method to use it as an 8-15mm fisheye zoom on MFT.

Part list – missing the zoom gear

What you need are the following:

  • Canon EF 8-15mm f/4L fisheye USM
  • Metabones Smart Adapter MB_EF_m43_BT2 or Viltrox EF-M1 Adapter
  • A 3D printed gear extension ring
  • Nauticam C-815Z zoom gear
  • Nauticam 36064 N85 to N120 34.7mm port adapter with knob
  • Nauticam 21135 35mm extension ring with lock
  • Nauticam 18810 N120 140mm optical glass fisheye port

The assembly is quite complicated as the lens won’t fit through the N85 port. It starts with inserting the camera with no lens in the housing.

GH5 body only assembly
Camera in housing without port

The next step is to fit the port adapter

Attach N85 N120 Metabones adapter

Then we need to prepare the lens with the smart adapter once removed the tripod mount part.

Canon 8-15 on Metabones Smart Adapter IV

As the port is designed for the speed booster the lens will be few mm off therefore the gear will not grip. Wolfgang has devised a simple adapter to make it work.

gear extension ring
Zoom gear on lens

This shifts the gear backwards allowing to grip on the knob.

3D design is here

Lens inserted on housing

Looking at nauticam port chart an extension ring of 30mm is recommended for the speedbooster and now we have extra 5mm in length Wolfgang uses a 35mm extension. however looking at the lens entrance pupil I have concluded that 30mm will be actually better positioned. Nauticam have confirmed there won’t be performance differences. You need to secure the ring on the dome before final assembly.

Fisheye dome and extension
Full assembly top view
Side front view

The rig looks bigger than the 4.33 dome but the size of the GH5 housing is quite proportionate. It will look bigger on a traditional small size non clam style housing.

The disassembly will be made again in 3 steps.

Disassembly

I am not particularly interested in the 1.4x teleconverter version consider that once zoomed in to 15mm the lens is horizontally narrower than a 12mm native lens so there is no requirement for the teleconverter at all.

This table gives you an idea of the working range compared to a rectilinear lens along the horizontal axis as diagonal is not a fair comparison. The lens is very effective at 8-10mm where any rectilinear would do bad then overlaps with an 8-18mm lens. The choice of lens would be dictated by the need to have or not straight lines. The range from 13mm is particularly useful for sharks and fish that do not come that close.

Focal lengthHorizontalVerticalDiagonalHorizontal Linear EqWidthHeightDiagonal
8130.995.9170.217.31321.64
9114.984.7147.8
10102.575.9131.06.9
1192.668.7117.88.3
1284.562.9107.29.5
1377.757.998.410.8
1472.053.790.911.9
1567.050.184.613.0

Wolfgang has provided me with some shots that illustrate how versatile is this set up.

8mm end surface shot
Caves 8mm
15mm end close up
Dolphins at 15mm
Diver close up at 8mm
Snell windows 8mm
Robust ghost pipefish @15mm

As you can see you can even shoot a robust ghost pipefish!

The contrast of the glass dome is great and the optical quality is excellent. On my GH5 body there is uncorrected chromatic aberration that you can remove in one click. Furthermore lens profiles are available to de-fish images and make them rectilinear should you want to do so.

I would like to thank Wolfgang for being available for questions for providing the 3D print and the images that are featured here on this post.

If you can’t print 3D and need an adapter ring I can sell you one for £7 plus shipping contact me for arrangements.

Amazon links UK

Canon EF 8-15 mm f/4 fisheye USM lens

Viltrox EF-M1 Mount Adapter

Note: it is possible to use a Metabones Speed Booster Ultra in combination with a Tokina 10-17mm zoom fisheye and a smaller 4.33″ acrylic dome.

UK Cost of the canon option: £3,076

Uk Cost of the Tokina option: £2,111

However if you add the glass dome back

UK Cost of Tokina with glass dome: £2,615

The gap is £461 and if you go for a Vitrox adapter (would not recommend for the speedbooster) the difference on a comparable basis is £176 which for me does not make sense as the Canon optics are far superior.

So I would say either Tokina in acrylic for the cost conscious or Canon in glass for those looking for the ultimate optical quality.

Nauticam bayonet mount for wet lenses

Nauticam entered the wet lenses market with their SMC close up wet lens that was optimized for DSLR.

Then it released the CMC compact macro converter for compact cameras and micro four thirds and finally the Wet Wide Angle Lens I that is compatible with compacts, micro four thirds and also full frame cameras with 28mm equivalent lens.

Up to now all lenses were using the traditional M67 mount as most of the lenses, even the close up ones, are pretty heavy this means going for the dive with the same lens. Nauticam has developed the flip diopter adapter for flat ports to overcome this issue.

Flip Diopter on Nauticam RX100 IV
Flip Diopter on Nauticam RX100 IV

The flip diopter is a good solution for micro four thirds and DSLR but looks rather cumbersome on compacts as the image shows.

I asked Nauticam for a bayonet adapter and specifically if they could develop something for the Inon LD bayonet system that so far has been the reference for wet lenses for compacts and micro four thirds cameras.

LD mount converter on RX100 IV
LD mount converter on RX100 IV

Edward told me that due to the fact that the WWL-1 lens rear element is so large the Inon LD system was not an option so they went off and developed their own system.

M67 bayonet mount converter
M67 bayonet mount converter

I would like to thank Nauticam again for making those parts available before general availability.

Looking a bit closer to it you can see that due to the specific construction with two concentric rings you need a special tool to apply the adapter on the port.

M67 bayonet mount converter The large item is to attache the mount to the port
M67 bayonet mount converter
The large item is to attache the mount to the port

Obviously as the Nauticam lenses use an M67 thread new adapter needed to be developed.

Mount converter for CMC/SMC
Mount converter for CMC/SMC

Nauticam does not use ABS plastic and uses aluminum for all their parts.

Now that the items have a bayonet adapter there is a need for a lens holder to put on the arms.

Lens holder looks too big for a 5" arm segment
Lens holder looks too big for a 5″ arm segment

The lens holder is too big for a standard 5″ segment but looks in proportion with a longer segment.

Lens holder on 8" arm segment
Lens holder on 8″ arm segment

The adapter is larger than the LD mount and a bit big for compacts to the point that even with a tray the adapter tips the rig back.

Another challenge is that this system is designed for Nauticam lens that have protruding rear element so when used with standard lenses there is a gap between the port and the wet lens that can be counter productive, not the end of the world and frankly the Inon system has the same problem. This however means that if you wanted to use this system with a different wide angle wet lens this would be suboptimal.

I am waiting for Nauticam to ship me back the WWL-1 so I can show how that lens performs on this system.

Another observation of course is that if you use this system for wide angle the super heavy WWL-1 and the fact that the adapter only works on a normal segment means your rig will be very heavy in water. I am going to discuss with Nauticam the possibility to have the adapter on a float arm however their carbon arms do not have any mounting point to be used.

Stay tuned for a full review of this adapter with the new 29 macro port that looks very promising for video.

Guide to Micro Four Third Lens and Port selection for Underwater Photography

Micro Four Thirds camera have become increasingly popular for underwater photography especially for macro and close up subjects in the last years.

The micro four thirds standard is shared between Olympus and Panasonic however in terms of underwater use Olympus is on the forefront and in fact it produces their own poly-carbonate housings for all new models.

Nauticam has their own N85 port system for micro four third that as of today has 10 different ports, two extension rings and a large number of focus and zoom gear. You can also buy cheaper 3D printed gear to save a few bucks for majority of lenses.

The other interesting characteristic of the micro four third market is that since the introduction of the 16 megapixels sensors, there have not been any substantial improvement in image quality among the various model, we will have to see what the 20 megapixels sensor of the Panasonic GX8 will bring but all in all the camera choice is mostly one of ergonomics and features.

Olympus is quickly clearing stock but you can still found the OM-D E-M10 out there for £478 with kit lens. The might Panasonic GX7 is now £433 with kit lens. In US you can find both the Panasonic GX7 and the Olympus O-MD E-M100 for $599 with kit lens.

The Panasonic LX100 sets you at £589 in UK and $698 in US really if you are into photography and want a micro four third it is a no-brainer you get a previous version GX7 or OMD EM10 (as you can’t find an EM5 new) and happy days.

Now that you have committed to the camera you are into lenses and ports for underwater use. The Olympus housing costs on average 25% less than Nauticam and has less choice of ports for me really is not appealing so let’s assume we are into Nauticam. What lens or port to choose?

Both the Olympus 14-42 EZ and the Panasonic 14-42 II fit in the Nauticam 35 port and require 3D printed gear to operate. The flat port will be OK for fish portrait and if you already have wet lenses it is definitely recommended to buy the 35 macro port first especially if you shoot video.

Nauticam Macro 35 port
Nauticam Macro 35 port

Deepshot zoom gear on the GX7
Deepshot zoom gear on the GX7

Both lenses do need a wet diopter to shoot small subjects in water as the smallest frame is around 7-9 cm width otherwise.

If you come from a compact camera most likely you have wet lenses and therefore you can re-use them for close range work.

Obviously a flat port for a 28mm equivalent lens is not a solution for wide angle.

First of all at the wide end due to water magnification the angle of view is the same of a 37mm lens which gives a field of view of just 60 degrees. The other point is that you really want to shoot at the tele end to avoid cluttering the background too much and typically in water those kit lenses work only at over 25cm from the port which means they are only good for medium size fish portraits.

The sequence of shots shows that even if the size of the cup is pretty much the same it looks much better at long focal length.

Cup at 28mm
Cup at 28mm

At 28mm you can see the glass in the frame on the right side and the window in the background.

Cup at 50mm
Cup at 50mm

At 50mm with similar size of the cup the glass is half gone and the cup looks nicer less of the window is showing.

Cup at 42mm
Cup at 42mm

at 42mm the glass is nearly out of the frames and the windows in the background is gone.

If you did not have any wet diopters the first additional lens and port to get will depend on your subjects, if you shoot macro you need a macro lens, if you shoot close focus wide angle you need a fisheye lens it is as simple as that.

Olympus 60mm and Nauticam 65 Port (35 + 30 mini extension)
Olympus 60mm and Nauticam 65 Port (35 + 30 mini extension)

The Olympus macro 60mm is the preferred lens combined with the Nauticam 65 port (or 35 port plus 30 extension). This lens can focus 10cm from the port and therefore can be used also in low visibility. Really there is no reason to use the more expensive 45mm Leica lens. The 60mm is great also for portrait work of small fish.

Cup at 60mm
Cup at 60mm

The same shot of the cup at 60mm means we now say goodbye to the glass due to the reduced angle of view. The increased working distance also means a more pleasant perspective.

I hope this explains why for underwater macro we want to shoot with long focal length as this helps us to get rid of the background so a longer lens will always be better than a zoom lens with a wet diopter and it will also give more magnification in case you have really small subjects.

If macro is not your priority and you are happy with kit lens and diopter for wide angle the first option is the Panasonic 8mm fisheye, that since the introduction of the Olympus 8mm pro fisheye has also reduced in price.

Panasonic 8mm Fisheye and Nauticam 4.33
Panasonic 8mm Fisheye and Nauticam 4.33″ Dome Port

This lens works best with the Nauticam 4.33″ dome port. If you shoot in 3:2 format you can also try the Nauticam 3.5″ wide angle port that makes the set up even smaller however there are no substantial improvements on how close you can get as the strobes will fail to lit properly anything that is right on the port without risking to see the strobe lights in the frame. The 3.5″ port will vignette in 4:3 image format though some users report success.

EDIT 7 September 2015

The issue of the 3.5″ port has been put forward again with the argument that it allows you to get closer than the 4.33″ dome with the Olympus EM5. I do not have an EM5 but I do have a Panasonic GX7 the end of the dome is 9 cm away from the focus point and this means that the dome allows you to get closer than the minimum focus distance of the lens of 10cm. As the dome is shorter than the minimum focus distance on the GX7 the 3.5″ port does not bring any benefit. Furthermore as trays are in general around 30cm you need to be around 15 cm anyway to be able to illuminate the subject properly with edge lighting, in virtue of this I remain convinced that the 3.5″ port does not bring any real benefit except is a bit cheaper and may vignette.

So your starter kit if you upgrade from compacts of you own a close up lens is Macro 35 port followed by Olympus 60mm and mini extension ring 30 or Panasonic fisheye 8mm with 4.33′ dome for wide angle.

Wide angle zoom lens are less of a priority but the choice is between the Olympus 9-18mm and the Panasonic 7-14mm bearing in mind that if you had a wet wide angle lens you may not even bother getting an intermediate wide angle zoom lens.

That’s all for now hopefully this will cover most of the ground for all new micro four third underwater shooters with kit lenses!

Red Sea Workshop with Alex Mustard – Part III Sunbursts and the Caves

On the early morning of Day 3 we left the Thistlegorm for Ras Mohammed.

Dive number one would have been on Ras Zahatar, this dive site has some great opportunities for sunburst shots in the early morning but generally not a lot of fish as the location is quite sheltered and there is never too much current that I recall.

There are also some very nice gorgonian fans around the 22 meters mark however as soon as we jumped in it was clear it was going to be a competition to have some of it. Besides not having made clear arrangements for modelling with anyone it was going to be a quite technical session.

Since the arrival of digital sunburst shots have proven to be problematic for DSLR and mirrorless users. The issue is with the sunball itself there is an issue of highlights when you try to shoot a coral reef with the sunball in the frame and you are using a shutter speed of 1/250th or 1/320th that are typical DSLR sync speed.

Mirrorless cameras do even worst as usually the base ISO starts at 200 that makes it really difficult to capture this shot.

So the key is to put the sun behind something or have the rays in the frame but not the sunball. Something like this to give an idea

Soft Coral
The sunball just behind the soft coral gives a nice glow so even with a 1/250th shutter and f/8 at ISO 100 is possible to capture a captivating scene.

Few meters away there is a gorgonian with a red soft coral on top that is really exciting to see at naked eye. To me this reminds of a frogfish head profile. It is impossible to capture this scene at 1/250th f/11 ISO 100 it is just too bright you need to reduce at least two stops to get the sun properly however if you did that on a DSLR you would be shooting at f/22 and it is near to impossible to properly illuminate the coral with your strobes at the distance required.

That is where our RX100 compact comes to help as you can sync your strobes at 1/2000th this should give a black background around the coral, the coral properly lit in the foreground as I had the strobes at full power and the sunball in the frame as well without too  many glowing highlights, this is the resulting shot
Sunburst
Note that a model would not be visible in this shot and fish would be colorful only in front of the coral or close by anyway as discussed there was not fish to model for me and this was just technical entertainment. The image is quite strong but the lack of fish makes it less interesting.

I found some cooperating clown fish on this dive however they are those with the dark eyes so despite the eye contact the shot is not as strong as it would be with a better subject
Look Right in
This is taken with a single Inon UCL-165 a luxury of having a compact and being able to wide and macro on the same dive.

Anyway I was happy with the sunburst in the frame so that dive was well worth it.

Dive two was at Jackfish Alley, and proved to be the most entertaining of the day. Who knows this dive sites knows that there are two caves, the first is wide but very dark and at certain times has turtles inside. The second is really narrow but has few cracks the provides a cathedral light effect best experienced in other site of the Southern Red Sea. Anyway we got the briefing and Dr Mustard was going to be marshaling the queue of photographer in cave number 2.

Being familiar with the dive site I knew that cave 2 is quite narrow even for normal diver let alone this big troupe of photographer so I mounted my tripod legs on the tray with a view of working in cave 1 and then move to cave 2 after the chaos was over.

Well it was real chaos as you can see from the feature image divers in the way, bubbles silt, in short it was a mess.

So I spent some time in cave number 1, this cave is really dark and you can’t really shoot handheld. In cave number 2 you can use speeds of 1/25 or 1/30 up your ISO  and put the camera on a rock whilst you take the shot, in cave 1 this results in darkness unless you go to 4 digits ISO.

So my shot on cave 1 is taken at 0.4″ f/4 ISO100.
1st Cave@Jackfish Alley
The shot is taken with the camera on the tripod using self shoot so that there is no shake from the hand pressing the shutter, this means the picture is sharp as it can be and due to low ISO also very noise free. Unfortunately fish did not feature in the shot so this is again a fairly technical shot that is not as strong as it could be.

There were other interesting opportunities in smaller cracks like this one that I like quite a lot though is not a sensational shot.
Window
I like the fact that the two snappers are one silhouette and the other full color.

After the shambles of dive 2 we moved to Shark Reef where the first dive was really more to acclimatise with the site so won’t bore you with the pictures for that dive.

In the evening the sunset dive was at Ras Katy that would have been our regular evening spot from there onwards.

On day one I was playing with dapple light and reflections

Dapple at Ras Katy
Dapple at Ras Katy

 

This is me taking the shot above

Interceptor Shooting Dapple

As you can see I am nearly at the surface thanks to Damo for this picture.

Near surface can also be interesting like in this case

Test Shots dapple Reflections
Test Shots dapple Reflections

Here you can see part of the Snell windows together with the reef and the sunball. This shot can be made much more interesting if you focus on the reflections.

Anyway this was the end of day 3 of diving next post will be about Shark Reef and the schools of fish.

Snell Windows with your compact camera

Snell’s window is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees [From Martin Edge: Underwater Photography].

During my last Gapapagos Trip there was a bit of an issue in terms of photography subjects, in essence most of them were fairly big like sharks, turtles, eagle rays and of course divers.

After a while shooting or trying to shoot those uncooperative models I had the idea to try and do different things such as silhouettes and Snell windows. You can also combine both as we did in this case.

So what do you need to shoot image of a Snell window:

1. A lens that is wide enough, you need a bit more than 96º field of view to take the whole window

2. Something interesting on the surface (if you have calm water you can see right through)

3. An interesting subject as silhouette to contrast the clear water if there are waves and you can’t easily see through.

A normal flat wet wide angle lens for our compact camera is not sufficient to capture the snell’s windows as in this example

Safety Stop
Divers at Safety Stop – taken with Inon UWL-H100 at wide end

Take into account that even with a fisheye lens on an SLR you can’t capture the Snell’s window on the vertical axis as the field of view will fall short a few degrees.

With a compact camera a semifisheye lens will capture the edges of the window if correctly aimed on the diagonal and horizontal dimensions and will fall short on the vertical.

Snell to boat
Snell window divers exiting the water or ascending – UWL-04 semi fisheye lens Canon S95

I am reporting some of the calculated field of view for the most popular lenses at present for compact at 3:2 image format

Wet Lens Diag FOV Hor Fov Ver FOV Flare/Ghost
UWL-04 151.95 123.22 79.85 Ghost Possible
UWL-100 28AD 149.01 120.97 78.48 No
UWL-28 160.56 129.75 83.81 Both
UWL-H100 157.85 127.71 82.58 No

 

All those lenses will take a good Snell’s window I have not tried the UWL-28 and I have some concerns this lens may flare as it usually does but I do not know for sure.

Both the Inon lenses produce sharp images with no defect.

In general small sensor cameras like the Canon S series will work fine with the fix/idas lenses and should produce good results also of normal shots through the water.

Using the UWL-04 or UWL-28 with large sensor cameras like the RX100 there could be a sharpness issue a larger apertures so make sure you close that at f/8 or smaller instead of using the shutter speed to balance exposure if you want to go through the water.

Snell’s windows are uncommon with compact camera shooters as they are considered an advanced subject but they can be taken. I will be taking more in a next trip to Egypt I hope for calm surface conditions so I can go through the surface with the shots. Failing that this is very easy to practice in a swimming pool.

Have fun shooting snell’s windows!