580EX II focus assist beam not firing

So as the title says, my 580EX II's focus assist beam isn't firing. This is what I remember from Saturday. It was working with a set of eneloops that I changed out when they died and then next thing I noticed, it wasn't firing.

In camera settings have it as on and I couldn't check cfn settings on the flash as they're all numbered and I didn't have a manual on me.

Any idea what I'm doing wrong? Is the AF assist FuBaReD? It's in good condition and hasn't been abused. Of course, it was acting fussy all night and would randomly fire...

Your AF needs to be in One Shot mode (not AI Servo).

Same thing happened to me until I realized exactly what Curtis said.

But I have another question along these lines.

I once asked why the AF beam doesn't work in AI Servo and was told, to my chagrin, that then I'd get the beam pattern showing up in the pics. Okay, I buy that.

But with the main knock on the 1DIV being it's seeming "inability" to focus on low contrast subjects in poor lighting, why can't the beam work in AI Servo, and then instantly turn off before the shutter is completely pressed? Doesn't seem like a hard thing to do to me. But then, what the heck do I know.

me

Curtis N wrote in post #13609232
Your AF needs to be in One Shot mode (not AI Servo).

*facepalm* Yep. That would be it.

MDJAK wrote in post #13609250
Same thing happened to me until I realized exactly what Curtis said.

But I have another question along these lines.

I once asked why the AF beam doesn't work in AI Servo and was told, to my chagrin, that then I'd get the beam pattern showing up in the pics. Okay, I buy that.

But with the main knock on the 1DIV being it's seeming "inability" to focus on low contrast subjects in poor lighting, why can't the beam work in AI Servo, and then instantly turn off before the shutter is completely pressed? Doesn't seem like a hard thing to do to me. But then, what the heck do I know.

me

Please, 5D MKII in a dark club. Now that's inability to focus right there

MDJAK wrote in post #13609250
I once asked why the AF beam doesn't work in AI Servo

This is not an official Canon answer, but my guess is it would burn through your batteries pretty quick.