3 Custom Function SettingsN300C.Fn III: Autofocus/DriveC.Fn-4 AF-assist beam firingThe settings to specify whether or not the AF-assist beam is emitted by thecamera’s built-in flash or by an external, EOS-dedicated Speedlite can beset.0: EnableIf necessary, the AF-assist beam will be emitted by the built-in flash orexternal Speedlite.1: DisableThe AF-assist beam is not emitted.2: Enable external flash onlyIf an external Speedlite is attached, it will emit the AF-assist beam whennecessary. The camera’s built-in flash will not fire the AF-assist beam.3: IR AF assist beam onlyAmong external Speedlites, only those which have an infrared AF-assistbeam will be able to emit the beam. This prevents any Speedlite whichuses a series of intermittent flashes (like the built-in flash) from firing theAF-assist beam.With an EX-series Speedlite equipped with an LED light, the LED lightwill not automatically turn on for AF-assist.C.Fn-5 Mirror lockup0: Disable1: EnablePrevents mechanical vibrations in the camera caused by the reflexmirror action (mirror shock), which can disturb shooting with supertelephoto lenses or close-up (macro) shooting. See page 142 for themirror lockup procedure.If the external Speedlite’s [AF-assist beam firing] Custom Function is setto [Disabled], the Speedlite will not emit the AF-assist beam even if thecamera’s C.Fn-4 is set to 0, 2, or 3.
