Canon 17-40 and 16-35 Zooms
By Stephen Frink
I have been having some questions about corner resolution when using the Canon 16-35mm zoom and 17-40mm zoom in the Seacam housing. First, let me advise that the same zoom gear fits either lens, and the preferred port combination is the superdome and PVL35; or fisheye port and PVL35. I have not found a good combination of port extension to work with the wide port and therefore do not recommend with wide port.
However, there are other considerations besides which port and extension to use when evaluating wide angle zooms for underwater use. It is prudent to know what a lens does topside first.
To that end I have tested both the 16-35 and 17-40 with a full frame Canon EOS1Ds under identical topside conditions. The tests were at F-4, aperture priority automatic. The auto-focus is always in the center of the frame, and specifically on the diagonal image on the "sec" mark on the second exposure slate. Obviously results, both topside and underwater, will be different when using these lenses with a 1.3 chip as with the Canon EOS1D MkII.
Not all exposure slates are the same exact color, so variations therein cannot be attributed to the lenses.
My personal take on the test is that the 17-40 is better in the corners, but does vignette. The corners are darker than the center of the frame. The 16-35 is a bit wider, and is F-2.8 instead of the F-4 of the 17-40. Both good things. But there are clearly aberrations in terms of corner resolution.
What does this mean for underwater imaging? I think either lens is fine for reef scenics generally on the same plane. Both lenses will handle blue water subjects in the center of the frame. But for a composition that includes a subject focused sharply in the center of the frame, and reef visible closer to the camera in the corners, a potential combination of soft focus and spherical aberration should be expected. Probably more so with the 16-35 than the 17-40.
There are specific characteristics of certain lenses that port choices cannot change. What you have topside is what you'll have underwater, in a best case scenario.
Sometimes there are tradeoffs. The wide angle zooms are tremendously convenient, but may not be as sharp in the corners are certain other prime lenses.
In the Nikon family, the 17-35mm is similarly difficult to house. There are some issues with the 12-24mm in the corners topside as well, but overall in most imaging situations, it seems a good performer underwater.
16-35 photos for comparison:
17-40 photos for comparison:
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