Sample @17mm at f/8 with 170mm dome, NO FILTER . Note de lack of sharpness in the corners

Mitigating that issue

Since now, the main solution was to mount big domes and use small apertures. Nowadays, with digital cameras offering an amazing amount of detail, if we want to have decent corner sharpness we should use domes around 9” in diameter (of course with the right extension ring attached between the dome and the housing, and the extension length depends also on how long is the lens and where nodal point is!!) and f/stops over f/11 (You can find some articles and image samples on the net about that)

Of course, this question also depends on what you are shooting. For example if you photographing blue water animals where there is nothing but water in the corner of the image you can shoot at f/8 without being worried about corner aberrations. But if you shoot a reef scene the aberration will be quite noticeable if you not use f/11 or over.

These limitations are not negligible: shooting at f/11, or over, most of the time forces you to use high ISO. Nowadays digital cameras offer an excellent quality at high ISOs, and for sure it will improve, but in some cases it still means you’ll loose some quality because of the image noise. Other constrain is the use of a big dome. Today 230mm(9”) and 254 (10”) are the biggest domes available(3) and theses are really big and heavy pieces of gear, which could be an issue for travel photographers.

Solutions on the way

To solve this problem the better solution would be to use a wet corrector lens but this is not easy to get right now. There is some old wet corrector lens out there (thanks to Dr Alex Mustard who revealed this to all of us!), it was originally made by Zeiss for medium format cameras, like Hasselblad, and has been adapted to be used with modern dSLR and wide-angle lenses (you can learn more about that here: http://wetpixel.com/forums/index.php?showtopic=54681)

Some manufacturers have recently presented some possible solutions, but we must know that those solutions are, in my opinion, only half measures. One solution, presented by Nauticam, has created a wet lens that transforms a 28mm into an ultra-wide angle lens with around 130º. It seems to even work well with full-frame cameras like Sony A7 series. We must know that this lens transform the image into a sort of semi-fisheye lens with spherical distortion, so the image would not be rectilinear when lens attached.

Another way to correct the corner aberration, used mostly in the past, was to attach diopter filter in front of the lens. The more used filters were 2x, 3x or 4x, and the strength of the filter was directly related to the diameter of the dome used(4). The downsides of this method were: a reduction of the field of view, and a pincushion distortion created on the image. Both effects become stronger depending on the strength of the lens diopter, so a 4X lens creates a bigger pincushion distortion and reduces more the FOV than a 2X filter. In my opinion the image is more pleasant when there is no dioptre filter attached.

The Japanese manufacturer Sea&Sea has just released, IMHO, the more interesting solution available to date. They have created a filter that corrects the corner aberration without any other substantial effect in other parts of the image. The filter promises 2 f/stops improvement over images taken without it. The best quality is still obtained using a big dome… the bigger the best!!

I have been doing some test in a pool, and in the wild, with different domes sizes (200mm, 210mm and 230mm diameter). I have obtained the best quality with the 230mm dome with right extension (size depending on the lens). I have tried different lenses like the Canon 17-40mm f4 and the Canon 16-35mm f4 IS. The last one is a modern lens designed for high-resolution digital sensors and offers better corner sharpness on land when compared with the 17-40mm. As expected I have obtained better corner results with the Canon 16-35mm f4 IS.

Test conclusions are:

– The Sea&sea Internal Correction Lens offers a substantial quality improvement on corner of the image.

– In some cases the improvement is almost the claimed 2 f/stops.

– The lens corrects also the barrel distortion and turns it into a very light pincushion distortion.

– The improvement is more significant when using modern lenses that also offer better quality in the image corners when used on land.

– The best quality is obtained when using the filter behind a big dome (9”) and the right extension ring. With the Canon 16-35mm f4 IS, I have used and 70mm extension ring (the right extension can be different depending on housing manufacturer).

– To my taste at f/8 the corner quality is quite good and very good at f/11. At f/5,6 the quality is good enough for blue water subjects. (Note: the focus point has always been de “C” in the image centre)

The lens is not cheap (around 350€) but, in my opinion, is a good investment if you want to improve the quality of your images, mostly if you consider that the total amount you have spent on your photo gear. The lens is available in two diameters: 77mm and 82mm

You can get me more info at: http://www.seaandsea.jp/products/digital_slr/accessory/lenses/

(1) I will discard 14mm lenses (or wider like Canon 11-24mm, recently released!) because their front glass is spherical and it is not possible to screw a normal filter

(2) Zooms used at the wider side

(3) There are some bigger ones that start to appear but only for special purposes like split shots!

(4) A smaller dome needs a stronger diopter filter.

Watch the following samples at different F/stops:

UPDATE (30-11-2005)

I have received requests to add filter test images with smaller domes. For now I add some pictures with a 200mm dome. In short I will try to post some test with a 170mm dome. Images were taken with the Canon 17-40mm which offers less corner sharpness than the 16-35 f4 IS (which I borrowed for the first test). This lack of sharpness, along with the use of smaller dome, may be the reasons of more evident aberrations than with 16-35 f4 IS. I attach only f/8 and f/11 results, f/stops where improvement starts to be obvious.

]]> https://uwaterphoto.com/?feed=rss2&p=839 0 https://uwaterphoto.com/?p=821 https://uwaterphoto.com/?p=821#comments Mon, 28 Sep 2015 12:07:32 +0000 http://uwaterphoto.com/?p=821 Some days ago I had the opportunity to try the new Canon 5DsR (thanks to Foto Casanovas). This is the new Canon flagship, in terms of resolution, with its new 50Mpixels sensor and the Digic 6 processor. The Japanese manufacturer launched two versions, the 5Ds, with low pass filter, and the 5DsR, without low pass filter, which gives an extra sharpness at a cost of producing some moiré (quite uncommon underwater). Surprisingly the camera has exactly the same body than mythic, and already old, 5DmkIII, which allowed me to use the camera in an existing housing.

With only a couple of days with the camera I had not enough time to do an extensive test, so I decided to enjoy it and take some photos to know how it performs in a normal dive compared to the 5DmkIII. My main goal was to conclude if having 50Mpixels is a real advantage underwater, and if this huge resolution is at a cost of any constraint.

My friends at Foto Casanovas also lent me the new 16-35mm f4 IS lens. Canon, as well as other brands, are updating most of their lens catalog in order to being able to reproduce the big amount of detail that the new, and future, sensors are able to render. This lens is very sharp in the whole image and performs extremely well. I’ll talk about the lens at the end.

We did two dives and also took some pictures snorkelling. The first dive was with the 16-35mm f4 IS lens and the second one with the Canon 100mm Macro USM (the old, not L, version). We also took a couple of shots while snorkelling with the 8-15mm but without the right extension ring so I am not sure the lack of sharpness in the corners is because of that. As housing, we used the Isotta 5DmkIII with the 8” glass dome port and a M67ø Macro Port.

Wooowww!!! 50Mp is a lot!!

I knew that even before using the camera but when you zoom on the image, even on the LCD, you realize the big amount of detail it offers. To get such a big files, with that amount of detail, have some evident constraints. The amount of data you produce is huge (each RAW file has 70Mb), so you would need high capacity cards (the camera works with Compact flash and SD cards), high capacity hard disks and a computer able to manage big files like this (I had some problems editing the files with my 5 years old Macbook Pro).

You should know that you will need the best lens you can find out there because that sensor is capable to show you all the defects your old lenses have. It happened to me with my beloved 100mm macro USM; I always thought the lens was enough sharp and good and that I do not need to buy the new 100mm IS USM “L” lens, but of course if I would buy this camera I’d also need to update the lens. I never got amazing detail with the old one, even at f/8 or f/11considered the sharpest f-stops in a full frame camera. Diffraction, a problem which you even won’t consider with camera around 12Mp, it is more that evident when using the 5DsR at high f/stops. You can’t go over f/22, the lack of sharpness because of this optical phenomenon is quite visible. So, macro shooters should stay around f/8 and f/16 if you want to get the best results.

Some photographers say that such a big resolution gives you enough margin to crop and still have a huge image, and I even heard someone telling that since he uses this camera he has stopped to use magnifying lenses for macro, he crops and that’s all!! Putting aside ethical considerations, I prefer to avoid cropping if possible mostly because a big magnification together with such a huge resolution allows you to get an amount of detail that even your eye is not able to see and that you can explore during edition.

I am a wide-angle shooter, so it is in this terrain where I have found more advantages on shooting this resolution. Ten years ago I bought my first full-frame camera, the Canon 5D, a 12Mp camera that supposed a great leap forward then. Now, the 5DsR has 4 times its resolution, so you can divide a 5DsR picture in four and get 4 images the resolution the old 5D has. This is a good advantage when you photograph some shy and fast animal that won’t give you many opportunities to get the right shot, so even if you are not close enough to fill the frame (and most of the time you are not close enough!!) you can crop a little bit, have a better composition, and still have a high resolution image.

Speed

One the most significant improvements of the 5DsR, resolution aside, is the speed. The new processor, DIGIC 6, let’s you take 14 Raw pictures at 5 images/second burst rate (510 jpegs) with UDMA 7 cards. This is a lot of data!!!

The 5DS R also introduces three new in-camera crop-shooting modes – 1.3x (APS-H), 1.6x (APS-C) and 1:1 – with these crops being visible in the viewfinder. However, there’s no increase in frame rate using these options.

In terms of AF Speed the 5DsR uses the new system already implemented in the 7DmkII with some new features like the iTR (Intelligent Tracking and Recognition) which tracks faces and colour.

The ISO performance is quite good, although I have not taken pictures at very high sensitivity (maximum at 640iso). The noise presence seems very low for such a high resolution and close enough to cameras like the 5DmkIII.

The video capability does not represent an improvement over the 5DmkIII, in fact in some cases it has less functions (lack of audio jack, for example!). It is clear that Canon targeted this camera only for photographers.

Conclusion

The 5Ds and 5DsR are new “Queens of Resolution”. The amount of detail theses cameras can capture (mostly the 5DsR) is amazing and there is no other full-frame camera able to do that at this high level. The camera is also very good in terms of AF speed and precision, and the buffer is surprisingly fast for such a big amount of data. So, if you use the best lenses available, you have a very accurate technique and the conditions are good, you will get pictures with an incredible amount of detail and you’ll be able to print them very, very big… but, being said that the real question is: do we, underwater photographers, need that resolution? For me the answer is “you need the best conditions to take profit of that”. Why? Because in most cases, as underwater photographer, I won’t profit that resolution improvement. Unfortunately, underwater there are many factors that are conditioning the sharpness and details: water visibility, colour absorption, particles floating, lack of light, etc. So only in ideal situations, with lots of light in clear water and subjects that let you get close to them you will profit of this huge resolution. In fact, at least for me, with a 20 to 24 Mp should be enough and I’ll prefer a better iso performance, higher burst rate, higher dynamic range or more advanced video capabilities. But if you need high resolution this is the camera to go.

The 16-35mm F4L IS USM

The rectilinear ultra wide-angle lenses have been a real nightmare for underwater photographers, mostly for those using full-frame cameras, because of the lack of corner sharpness behind a dome. Most of those lenses only perform well when you use a huge dome (9,3” dome is the best) and when you stop down over f/8, getting the best results over from f/11.

The new 16-35mm F4L IS USM it is a great performer, not only on land, but also underwater. I have been really surprised of the good corner performance even using an 8” dome port (mostly at f/11 and over). Stabilization works very good also, and it is useful with a high-resolution camera like 5DsR. This is a real great lens.