http://gdlp01.c-wss.com/gds/2/090000...kIII_HG_EN.pdf

Strona 107:

If focus cannot be achieved with the external Speedlite’s AF-assist beam, select the center AF point. With some external Speedlites, autofocus with AF-assist beam works only with the center AF point.
Strona 165:

C.Fn II -6 Flash firing
Enables or disables the firing of an external flash or non-Canon flash connected to the PC terminal.
0: Enable
1: Disable
Convenient when you want to use the external flash’s AF-assist beam only. Note that whether or not the AF-assist beam is emitted is dependent on the C.Fn III -14 setting.
Strona 172:

C.Fn III -14 AF-assist beam firing
Enables or disables the EOS-dedicated Speedlite’s AF-assist beam.
0: Enable
The external Speedlite will emit the AF-assist beam when necessary.
1: Disable
http://gdlp01.c-wss.com/gds/4/030000...exii-en-zh.pdf

Strona 27:

C.Fn-08 AF-assist beam firing
0 Enabled
1 Disabled
Strona 49:

About the AF-Assist Beam
Under low-light or low-contrast conditions, the built-in AF-assist beam will be emitted automatically to make it easier to autofocus. The AF-assist beam works with all EOS cameras. The AF-assist beam is compatible with 28mm and longer lenses. The effective range is shown below.

Position Effective Range
Center 0.6 - 10 m / 2.0 - 32.8 ft.
Periphery 0.6 - 5 m / 2.0 - 16.4 ft.
https://photography-on-the.net/forum...d.php?t=750965

shimmishim:

Hmm... I've never noticed before... at first I thought it was the flash (580EX II)... but I put it on the 5D Mark II and the AF beam fires.

I checked the custom functions on both the flash and camera (flash - CFn #8 is set to 0) and on the camera to make sure it was on... and it is.

Any idea as to why it won't work?

Thanks!
joeseph:

focus set to one-shot? if not it needs to be...
shimmishim:

wow... that was easy. I never knew that. i was in servo.

thanks!
XterraJohn:

I usually keep my focus in servo mode, and have it moved off of shutter button and onto the back button. Boy was I confused when I couldn't get the AF-assist to fire.