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 1

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.

Answer 2

You can put a flash on the camera that has a focus assistance beam. You can set the camera to not use the flash, but still use the focus assistance beam.

Answer 3

You can use the ST-E2 which gives you a focus assist beam without the bulk of a flash. http://www.amazon.co.uk/Canon-Speedlite-Wireless-Remote-Transmitter/dp/B0000C4G95/

There is further discussion on the matter here: http://www.jwz.org/blog/2009/11/alternatives-to-canon-speedlite-transmitter-st-e2/

Answer 4

In addition to focus assist mentioned by Guffa, some more ideas to try:

• Assuming lighting is uneven, anticipate when the dancers are in brighter areas - it will be easier to focus when there's more contrast. It will also work favorably for composition - brighter areas call for attention, so you want your subject there anyway.

• Compose with more room around dancers. It requires you to either shoot from further away or use a wider lens - both way you will be shooting closer to hyperfocal distance so focusing takes less adjusting and missed focus is not as noticeable.

• Use AI Servo focusing so the camera would try to keep focusing and be ready to shoot when you fully press shutter.

• When you get a well-focused photo, switch to manual focus and wait until the dancers return to same spot.