Edithburgh Sun Shots - Leafy In The Sunlight

Hello Matthias,

Thanks for visiting my photo-blog. I have only been diving with the Nexus housing for a few months but I think I have learned enough to be able to help.
I assume you already have read my post on the FP160 dome, http://robertrath.com/serendipity/archives/114-Nexus-Underwater-Housing-With-Canon-EF-17-40mm-Zoom.html

For the sake of these comments I will assume your 16-35mm USM II will behave similar to my 17-40mm. My first attempts using the 17-40 with the 40mm extension were very bad as expected for the smallest apertures. Increasing the extension to 60mm and using +2 diopter produces quite good results but at a loss of angle of view (now equivalent to 20- 50). This this is not the sweet spot. Attempting to use 70mm extention resulted in very bad vignetting which even +4 was not able to remove ( I did not try any higher as I wanted to avoid stacking diopters ).

The fundamental problem is that the FP160 dome (with no extension) is designed to work with 180deg FE lenes such as the Canon or Sigma 15mm FEs and relies on the FE optics to compensate for the misalignment of radius centre to entry pupil. This misalignment results in two effects, a) additional stretching of the peripheral image and b) some distortion in the shape of the focal plane (curved surface in the case of domes). FE lenses cope with these distortions very well where are rectilinear lenses do not. The advantage of the FP160 being shallow cut (ie not a full half hemisphere) is good close focus wide angle performance. A secondary problem is poor close focus performance with the 80mm dome radius.

For the 17-40mm lens to really work well and I assume the 16-35 USM II as well I feel that a larger diameter dome, at least 200mm ( addresses close focus performance issues ) and a more complete hemisphere ( addresses entry pupil alignment ) would be needed to get these rectilinear wide zoom lenses to work well. You will have to look at third party products such as http://zenunderwater.com
if you want to go down this route.

Hope this helps!

...Robert