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.
In addition to the set up required to use the Canon 8-15mm you need 3 additional items:
Kenko 1.4 Teleconverter
Canon 8-15+TC zoom gear
Extension ring N120 20mm
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.
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.
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.
With the rig assembled I made my way to the pool with the local diving club.
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.
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.
At close range you get the usual depth of field issues depending on where you focus but this is not a teleconverter issue.
For comparison a 15mm image without TC.
Obviously what is interesting it that you can zoom in here a set of shots at 16, 18, 21 mm.
Finishing up with the required selfie.
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.
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.
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.
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.
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.
This is the same target with the 4.33″ dome.
Side by side shows the difference in magnification.
If we look at the same detail we can see that the 140mm dome image detail is less blurred.
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,
The overall size of this dome means it is flush with the extension ring.
This is the overall rig with the amount of flotation in this image it is around 600 grams negative in fresh water.
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.
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.
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.
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.
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.
If you move your focus point a bit further in front the situation improves.
At this point I decided to get into the picture with a white balance slate.
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.
Now the depth of field is there although the detail in the centre is less sharp.
Moving the focus point makes the image a bit better.
Time to insert the diver in the frame.
Overall ok not amazing consider the dome is on the parts.
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.
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
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.
You need to stop down the lens to f/16 to start getting coverage for the edges.
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.
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:
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.
We can see that despite the edges are quite blurry this image is actually better than our flat target.
At f/11 the image is not perfect but we can see that most details off centre are not looking bad at all.
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.
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.
Let’s see how this goes. at f/11 we already get some better results.
At f/16 we get some additional improvement but is not as major as the original f/16
Looking at the other areas there are some minor improvements but generally less as we close down the aperture.
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.
We now reduce the aperture to 2cm which is more or less f/8
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.
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.
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.
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 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
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.
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.
As we can see the image is not too bad even in the close area but it is definitely better 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.
Here a detail crop the image is still fuzzy despite then focus is right on the spot. Depth of field is not the issue.
And finally we close down the aperture to f/11.
Crop at 100%
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.
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.
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.
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.
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.
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.
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.
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
The lens is absolutely pin sharp across the frame, I was shooting at f/5.6 in the 140 mm glass dome.
Performance remains stunning across the zoom range. I also tried few shots at 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
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.
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.
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.
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.
Horizontal Linear Eq
Wolfgang has provided me with some shots that illustrate how versatile is this set up.
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.
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.