Af-Assist Beam - Canon Speedlite 470EX-AI Instruction Manual

AF-Assist Beam

The AF-assist beam supports most of the EOS cameras' AF points. The
AF-assist beam covers the angle of view of 28 mm or longer lens focal
length, and its effective range (at 28 mm focal length) is approx. 0.7 - 10
m/2.3 - 32.8 ft. at the center in the viewfinder and approx. 1 - 5 m/3.3 -
16.4 ft. at the periphery (AF points other than the center AF point).
If a peripheral AF point is selected, or a wide-angle or telephoto lens is
used, achieving focus may be difficult with an EOS-dedicated, external
Speedlite's AF-assist beam. In such a case, use the center AF point or an
AF point close to the center.
During Live View shooting, the AF-assist beam is emitted even when the
AF method is set to [Quick mode].
AF-assist beam firing can be disabled (C.Fn-08, p.86).
The AF-assist beam type that uses intermittent flashes (a series of small
flashes) can be emitted (P.Fn-04, p.89).
E-TTL II/E-TTL Autoflash by Shooting Mode
When it is difficult to autofocus on the
subject in low-light or when contrast is
low during viewfinder shooting, the
infrared AF-assist beam built into the
flash is automatically emitted to help
autofocus.
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