Auto-focus assist question

[KaplanMultimedia]

I asked this question in the "People" category but no one seems to know the answer to the following question. I'm hoping some of the flash experts over here can help.

"Does anyone know if the auto-focus assist light from the 580EX flash only illuminates at very low light levels? The other night I took pictures in a room that was dimly lit but not extremely dark. I don't think the auto-focus assist light illuminated on many ot the shots, so sometimes my lens had a difficult time focusing and absolutely would not focus at all on a couple of shots. I'm guessing the room was lit just above the point to where the focus assist light would illuminate, so it didn't assist the camera in auto focusing".

Does my theory hold any water? :confused:

[PacAce]

Yes, that is correct. The AF assist light is only turned on when the lighting is low. If you had a problem with focus but the assist light did not turn on, then it might have been not because of low lighting but because the subject had too low a contrast for the AF to work right.

[Jon]

It's also possible (not sure on the 350D) to disable the AF-Assist beam. It's one of the custom functions. That's worth checking into.

[woffles]

Something else that can help get a focus sometimes is to turn your camera to portrait angle and it may pick up the focus and just flip back to normal and shoot.

[Raphael Emond]

Make sure you're in "One shot" mode, because it will not work in AI Servo

[KaplanMultimedia]

Something else that can help get a focus sometimes is to turn your camera to portrait angle and it may pick up the focus and just flip back to normal and shoot.

Thanks for your replies guys.

Woffles, I never heard this technique before. If it helps focusing when those difficult situations arise, then this is a great bit of information. Has anyone else heard of this and why would it help? Thanks.

[tim]

Something else that can help get a focus sometimes is to turn your camera to portrait angle and it may pick up the focus and just flip back to normal and shoot.

In theory this shouldn't make any difference if you use centre point focus, since that focus point on the 20D is a cross type sensor. In practice... i've no idea.

[KaplanMultimedia]

In theory this shouldn't make any difference if you use centre point focus, since that focus point on the 20D is a cross type sensor. In practice... i've no idea.

That's what I thought too. I also use the center point focus.