Specifically I am wondering if 100mm might be too long, such that little movements in the camera will result in changes to frame composition.
thanks in advance, damien
First of all, great job, Matti! Really shows some of the potential of the 20D...and it was your first dive trip with it!
Damien - the 100mm is definitely NOT too long. With the 20D's 1.6X crop factor, it acts like a 160mm, so you can shoot "super macro", and it is a GREAT lens, as long as you have decent light (which you certainly should most of the time Down Under!). Don't worry about it making composition difficult; it really doesn't. It's only drawback IMHO is in low light situations, where it sometimes has a hard time with AF ("hunts" a lot). You can solve that by using a manual focus ring, switching the AF function from the shutter (half-press) to the * button, and using AF only to get "in the neighborhood" of focus, and then use MF to fine tune it. That will NOT be necessary in nice daylight -- only late afternoon, night dives, etc. In short, the 100mm lens is easy to get used to, is a great, fast-focusing lens, and is quite affordable. You WON'T be sorry if you get one...in fact, you'll wonder how you ever lived without it! 