Primes vs zooms. Hmmm.
Zooms are nice, they do offer lots of flexibility u/w. However, I'd say most (80% ?) of the folks with them use them as a crutch.
By this, I mean that the purpose of a zoom is to allow you to select the perspective you want compositionally. This is wide-angle forground emphasis, or, long-lens combine forground with background. Or perhaps to allow wide-angle depth of field that combines forground and background, or, long-lens blow out the background focus.
After selecting the appropriate perspective, you use your feet (topside) or fins (U/W) to get the framing you want.
Unfortunately, most zoom users use it as a framing control, ignoring the perspective changes that happen as the lazily stay in place and rack the zoom in or out to frame the subject.
With a prime, you're forced to move around to frame, and this ain't neccessarily bad.
Other stuff about primes: you have to have a good idea of your subject material as you're selecting your lens for the dive. This is okay for me, I usually have a good idea of the shot(s) I'm shooting before hopping in the water anyway.
Generally, primes are faster and sharper. The faster is real nice through the viewfinder. I use the Nikkor 14mm 2.8 a lot, because my surface shooting makes me tend to rectilinear. There's those that love the 10.5 fisheye for underwater use, that's a personal preference thing, but again, fast and sharp.
If the tradeoff (being locked into a single perspective) is worth it for you, why wouldn't you shoot with a prime?
All the best, James
