Press to focus. Pick something very close to what you are trying to shoot, but brighter. Once the lens gets close to focusing, take finger off shutter and focus again. This will help minimize the lens racking all the way in and out.
I take it you have this lens and you've noticed it doing that as well - PITA isn't it - especially with the relatively stoneage AF system used on this lens - good tip though thanks.
I reckon then that if I combine separating AF and shutter buttons with this technique, I'll be pretty close to a complete solution.
As answered, diffraction. I find the image degrades noticeably after f/16. I choose f/22 as the start point to my compromise between depth of field and overall sharpness. But this depends on the distance to subject.
Thanks again - I'll have a play around in that range and do some comparison shots beyond f22 to find a compromise that works for me.
Camera shake only exists for the portion of an underwater image who's exposure is not provided by the strobes. It shouldn't be a problem for macro photography, unless you are very shallow with tons (kilos) of available light. In that case you would actually use a faster shutter speed and let the strobe provide more light. This gets rid of motion blur and provides better color.
Pretty much as I anticipated, but nice to know I'm on the right lines.