I guess I don't fully understand my options when it comes to focus modes (I use a Canon 20D and clearly have just started!). Can someone explain in simple terms what they do and which to use?

My Canon gives you the options of AI-Servo or One-Shot.

With One-Shot, the camera has to achieve focus lock or it will not let you take a photo. When the button is half pressed, the camera will achieve focus, lock, and then you can move it around without the camera refocusing.

With AI-Servo, the camera will be constinuously focusing when the button is half pressed. You can take a photo at any time, but no guarantee (sp??) that the shot will be in focus.

Cheers
James

Further re: When shooting in AI Servo mode (moving subjects), the camera needs to lock focus on the moving subject first. From that point as long as you keep your shutter halfway pressed, the camera will continue to make focus adjustments as you follow the fish. It may help in the low light environment of U/W to inititally aim and focus on a high contrast area of your subject to lock the foucs on then as you track your moving subject adjust your composition to your liking (without of course taking the focus area in your viewfinder off your subject).

I would suggest playing with your focus modes on land with a pet or better yet if you have kids, try it while they are playing outside.

Keith

That's a great idea Keith. The best way to familiarize yourself with the various focus modes and options in the camera is with a lot of practice. Since we are not underwater all the time, practicing on land helps a lot (or at least it helped me a lot).

Luiz

I like using One shot for fairly static subjects such as nudibrachs or CFWA images. One shot lets me focus on the Nudi's head then recompose so I don't have the area of focus solely in the middle of the image. Same thing for CFWA shots where I want to put the closest object in the image lower on the frame. Howver, if I have time, I will switch to the other focus points on the camera instead of doing the focus then recompose method.

AI Servo works great with moving fish, pygmy seahorses etc. If the focus engine of the Canon DSLRs are rated to follow a fast moving car, then a fish is well within the capabilities, though when you're trying to get a good shot of an anemone fish, you'd swear that wasn't the case! It would help if you let the camera find focus first on the subject for a second or two before firing. For pygmy seahorses, I find that AI servo is great because the tracking system compensates for the my slight bouyancy changes as I breathe in and out.

Stu

I have found that sometimes the AI Servo can keep focusing when you may not be completely still and cause you to miss the critical focus point. That is where I prefer the one shot as it locks focus and allows me to recompose without the focus point being adjusted.

AI Servo is great when used in conjunction with a * button that has been reprogrammed (via custom functions) to control autofocus. Just press the * button and the camera will hunt for focus. Release the * button and focus will lock.

-Brad

I have not desided which I prefer. I like the way one-sot will not let yoiu take an out of focus picture. But I also like Servo for quick moving targets.

No one has mentioned the third AF mode: AI AF. In this one, the camera switches between one shot and servo depending on the situation.