Autofocus?

I remember the days when we scoffed at the idea of auto focus - how could a machine possibly focus more accurately than the human eye, etc., etc.. Well, we've had to retract those ideas a long time since but it doesn't hurt to discuss the uses of auto focus. In my 1Ds2 auto focus is more accurate than my focusing, it's also a hell of a lot faster. I can't comment on it's follow focus abilities since I don't tend to do that kind of work but understand that it's pretty good.

That being the case, how come most of my lenses spend most of their time on manual focus, and does that mean the auto focus feature is a waste of time?

The problem for me with auto focus is I don't know for sure where it's focusing - sure I know which little focus rectangle lit up, but was it focusing on the branch in front or the one in back. My preferred use of auto focus is focus confirmation - ie. I turn off auto-focus on the lens, focus manually and usually but not always use the focus confirmation feature (in my camera the focus rectangle lights up) to confirm that I got the focus right. I may well move the camera so that it's clear what I'm focusing on. Occasionally I will use auto-focus when I can't see well enough - this is particularly true of wide angle f4 lenses. Sometimes I will auto focus, switch to manual move the camera and then take the picture (the half press of the shutter idea not working when I use a cable release).

Where I am relying on depth of field to cover a near far situation, the traditional rule of thumb is to focus 1/3 of the way from near to far to split the depth of field equally. As this happens to correspond to the centre of the image, if the near and far limits are equidistant from the edges of the image, then I just frame and focus. If not, then I aim for focus, switch to manual, and move the camera to the final framing.