The guy who said there is no such thing as a dumb question, never read this!

This evening, I hooked up my 70-200 with the 580EX. I tried to take pics, but it would not focus, or take the picture. I finally had to place it on manual focus in order to take the picture. Is this what it is suppose to do when the ambient light is too low to focus, not take a picture at all due to the auto-focus, even though I have a Speedlite attached? Thanks.

did it hunt or just not work at all?

If you are in AI Servo mode the flash will not emit the focus assist light. This could be the problem.

Hmmmm, I had the lens on AF, the lens would hunt for the target, but couldn't find it, so it just kept hunting and wouldn't take the pic. Now, AI servo, that is on the camera, right? I believe I had it set on one picture, I will check the 30D's settings now and see.

Joe

Rabidcow is right. You need to be in One shot for the AF-assist to work.

If you were in One shot, then we need more info to help you.

Ken

How is your C.Fn. 05 (AF-assist beam/Flash firing) set?

canonphotog wrote in post #2313508
Rabidcow is right. You need to be in One shot for the AF-assist to work.

You can be in One Shot or AI Focus - the AF assist beams emit in both these but not AI Servo.

Jim G wrote in post #2313521
You can be in One Shot or AI Focus - the AF assist beams emit in both these but not AI Servo.

You're right. Unfortunately it only emits once in AI Focus. As soon as the subject moves and the Camera starts tracking, it won't emit again. That's why AI focus is pretty much a useless mix of One shot and AI Servo.

Ken

I am in one shot mode. I've just changed lenses to the 17-40. I place my hand close to the lens so as to prevent it from auto-focusing, same thing, it won't take the shot. I can confirm, it is set on "one shot." Hmmmm. I will check the custom function modes..... yep, they are all set for "0" Since I am new to all of this, I was wondering if this was normal for it to refuse to take a picture if the auto focus can't focus. When I had it on the 70-200, I had normal indoor light on in the room, not bright, but not dark either> ?

Oh, and I have taken the 580EX off now, and I did not pop up the internal flash. Same problem, unless the camera can find an auto focus point, it will not take a picture. Geez!

did you have your ISO boosted and the camera at 2.8?

Was your focus assist light coming on from the flash?

Check C.fn-12 on your flash is set to 0. Set to 1 it will disable the AF-assist. The camera cannot override it.

Ken

I had the camera set on "P" and didn't change any settings. I have noticed this previously. The question I had, is it normal not to fire a picture if the auto-focus can't focus due to low light? I just got the 580EX. I put it on and nothing more. It worked fine with the 17-40, it was when I changed lenses to the 70-200 when it just began to hunt and would not take a picture until I set the lens on manual and then took the picture. The other test I did, was put my hand too clsoe to the 17-40, not allowing it to auto focus, smae result, no auto focus, no shot. This wouldn't seem to make sense, as I should be able to go into a completely dark room and take a picture with a flash. My guess is that in low light situations, we have to use manual focus then???? I am clueless as this is my first set-up so I have no knowledge base from which to draw, except you kind folks, and then, you might be asking questions beyond my ability to answer, such as was the 580 auto focus light coming on??? Sorry, long holiday and I have been really, really sick for the past 24 hours, vomiting and all, yuk!

canonphotog wrote in post #2313584
Check C.fn-12 on your flash is set to 0. Set to 1 it will disable the AF-assist. The camera cannot override it.

Ken

Will do, thanks. I am feeling horrible again, just has been 24 hours of hell for me, I thought I was coming out of it, but the spins are coming back. I will have to lay down for a while until the spins disappear. Not sure if I have the flu, food poisoning, whatever, but this is not fun to be sick like this. Thanks all.

Skrim17 wrote in post #2313564
did you have your ISO boosted and the camera at 2.8?

Neither of these things will affect its ability to autofocus.