Tuesday 15 July 2008

Autofocus assist beam

For further photography-related information check out my compendium of tips.

Autofocus relies upon the contrast between dark and light areas - this becomes more difficult to assess as light levels drop. My venerable Canon EOS 5 has a built-in AF-assist light which automatically switches on and beams a near infra-red light pattern from an LED to help the AF system. My Canon EOS 5D, however, must rely upon either a Speedlite or the ST-E2 Speedlite transmitter for AF-assist. I imagine the reason for this is that an in-built AF-assist light would struggle to cover the 9-point AF system 0f the 5D.

When I'm shooting a wedding it's generally around the first dance that I see the reassuring three vertical red lines of the AF-assist beam on my subjects. I can be shooting at f/2.8, or lower, and yet I rarely get a shot that is out of focus. Impressive stuff. Tips for focusing: I only use the more sensitive central focus point, to take advantage of both the f/5.6-sensitive cross-type sensors and f/2.8-sensitive vertical line sensors, and I look for areas of good contrast, such as the line between a white shirt and a dark jacket, to focus on. If you're going for a portrait shot, try and focus on eyelids or eyebrows.

In the 580EX II Speedlite manual it states that the AF-assist beam is compatible with lenses of focal length 28mm and longer, but it seems to work perfectly well at 24mm with my 24-70mm f/2.8 L zoom lens. The effective range of the light is 0.6 - 10 m at the central AF point, and 0.6 - 5 m at the outer points.

When shooting in low light the AF-assist beam fires twice - initially to calculate a focusing distance for the lens and then, secondly, to confirm that the subject is in focus before taking the shot. If the subject is moving too quickly, the focus distance will have altered between these two readings and focus lock cannot be achieved. For this reason the AF-assist beam is not compatible with AI servo - your camera must be in one-shot mode. For the tricky shot of the bride and groom walking down the aisle I have the camera in AI Servo mode only if the church is well-lit. In low-light I switch to one-shot mode to take advantage of the AF-assist beam and take plenty of shots to maximise my chances of getting one that is sharp (I always ask the couple to walk slowly beforehand).

As mentioned in an earlier post I use an Ee-S focusing screen to aid manual focus. This has no impact on the autofocus system but does affect the metering system (this is set with custom function 00). With lenses of aperture f/2.8, or lower, the viewfinder remains bright.

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