First of all, thanks Paul for taking time to explain all that (although, like David, it kinda makes my old synapses overheat trying to understand it!
)! That explanation DOES help, and in a situation where I was fairly sure I wouldn't want to do a split over/under, a diopter might be worth the trouble and cost.
In most cases, however, I'd probably be more likely to get as much as I could from the lens (i.e., after Stephen Frink has declared a certain ext. ring as being best for it in a Superdome), and if the edges are still soft, just crop them out. That's what I do now, and it's one of the advantages of having all these "extra" pixels on my 1Ds MkII; I can crop the daylights out of a pic and still have 8 or 9 megapixels left.
> Bruce, dont you have one of those Canon 24mm F1.4 lenses, too? Just don't get too close to your models, as in your latest shoot, and don't aim extremely up or down....<
Yes, like James, I do have a 24mm f/1.4 (that's what he and I were "pool testing" in the Bahamas in the photo I attached earlier in this thread), and I actually use it quite a bit. I actually find that in wide angle "situations" (e.g., big animals, plush reefs, etc.), I rotate between 3 lenses: 16-35mm, 15mm FE, and that 24mm. But your suggestion not to get too close to my "models" (which are usually non-human, so don't always cooperate) is not necessarily the most desirable option. In fact, it is contrary to one of the cardinal rules of uw photography (get as close as you possibly can to reduce the amount of water between you and your subject!). In general, I strive to get as close as I possibly can, and then use either a 15mm fisheye or the 16-35mm at the wide end. (I can't believe how many of my shots I look at and say, "Crap! I should have cut the distance between me and the subject in HALF!)
OTOH, since consensus seems to be that shooting the 16-35mm at 16mm is where the most "problems" are, and zooming in a little (say to around 24mm) yields better results, a person (especially one named Dave) might make the argument that it makes more sense to just use the 24mm f/1.4 for even those "close" shots. :glare:
But that gets back to the primary reason for using a zoom lens in the first place -- the flexibility to shoot a given subject/scene at the best focal length for THAT subject on THAT dive, rather than being forced to shoot it at a specific single focal length of a lens you happen to have on the camera (e.g., 15mm FE, 24mm, etc.) that dive. I like using the 16-35mm lens specifically because it lets me zoom in to 35mm for one shot, and then zoom out to 16mm when a huge beast swims right up to me. That's why I (and presumably so many others) occupy so many brain cells trying to get a lens like this to work as well as it possibly can uw!