How to speed up autofocus on moving subjects under low-light conditions?

Question

Since I am usually taking pictures under low light ambient conditions I have discovered the nifty 1.4 lenses (such as the Sigma 30mm/1.4 or the Canon 50mm/1.4 or also the Canon 85mm/1.8) for my Canon 550D. My main problem is not that there's not enough light to capture images, but that the autofocus – usually with one fixed AF point – is rather slow under those conditions, if it can focus at all.

My main subjects are dancers, who are quite naturally moving, so speed is an issue, especially with the narrow DOF the 1.4 or 1.8 aperture gives.

What can I do?

Answer

some ideas, based on the assumption that AF systems have it easier in areas with more light, detail and contrast:

• If the floor has some detail (e.g. it's made of wooden planks) focus easily on a well lighted area, lock focus and wait for dancers to get there. Then shoot repeatedly.

• Use a simple focus trap, it's basically like this: focus somewhere, press the shutter release button, keep it down until the camera gets the subject focused in the point you selected and actually shoots. How (and if) it's possible to do this depends on your camera. It's quite easy with Nikon DSLRs, it also should work with Pentax cameras if you know what you're doing (I tried fiddling with a K200D but couldn't get it to work the way I meant). I don't know about Canons.

• Most DSLRs have different types of AF sensors, often it's cross-type opposed to "just plain", cross-type being more accurate. You could try making sure you use those points (usually, on less-than-top-level cameras it's just one cross-type sensor, the center one) to get better AF, but I'm not sure this will make things faster.

• Focus on most detailed areas of the dancer's body (face, edge of the dress, lines where clothes of different colors meet).

• General advice: always look for detail or contrasts to speed up AF operation. Like a crack on a white wall, a bracelet or watch on someone's arm, the line between someone's hair and cheek skin, the lines beetween wooden planks on the floor.

Check more discussion of this question.