Looking for advice here. I've got a 5D MKIII with a Canon 17-40 f4L zoom in an Ikelite Housing with 8" dome port. I'm headed to the Galapagos Islands in a couple of months. I was pretty happy with the 17-40 when shooting in Bonaire last year, but since Galapagos is such a special destination, I am thinking hard about what to shoot.
Obviously, the advantage of the 17-40 for the Galapagos is that I can zoom, and the subjects that I want to shoot in Galapagos are big (whale sharks, schooling rays, hammerheads, etc). I am counting on the great low light resolution of the 5D MKIII to allow me to push the ISO up and not worry about the f4 and higher apertures. I know some people complain about vignetting and distortion with the 17-40 when it is as wide as possible, but that does not bother me. I kept it around 19mm max wide previously and was happy.
However, I've seen some nice shots with the 15mm lenses from both Sigma and Canon, and I'm looking for input from anyone that has compared them to the 17-40. I'm worried that if I switch to a 15mm that I may lose some shots when I can't get as close (where I would miss the 40mm end).
Or, I might track down a Canon 16-35 f2.8 version 1 if people point me strongly in that direction. The newer 16-35 v2 won't fit in an Ikelite housing/port due to its size.
Thanks in advance for any input, experiences, etc.