Canon SPEEDLITE EL-1 Advanced User's Manual page 46

AF-Assist Beam
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 automatically flashes to
help autofocus.
The AF-assist beam supports 28 mm and longer focal lengths of the lens and its effective
range (at 28 mm focal length) is approx. 0.6 - 10 m / 2.0 - 32.8 ft. at the center in the
viewfinder and approx. 0.6 - 5 m / 2.0 - 16.4 ft. at the periphery (AF points other than the
center AF point).
Caution
If a peripheral AF point 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.
Note
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).
The AF-assist beam can be projected using the intermittent flash firing method
(P.Fn-01).
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