5DM2 won't AF when using just a modeling light

I am a fairly new owner of a 5DM2 and am having an issue with low-light focusing, particularly in my home studio.

When I turn off all ambient lights (so that my only light source is the modeling lamp on my one strobe) the camera won't focus. I am using a 24-70L af f/11 with center point focusing. It seems that that the modeling lamp does not produce enough light (on the subject) to allow the AF to work properly.

Any suggestions on getting around this? I never had this problem with my previous body (40D) in the same studio environment.

If I had known this was going to be a problem I would have never upgraded from my trusty 40D.

I have no interest in MF because I photograph small kids that move around a lot.

using centerpoint?

john_galt wrote in post #15445868
using centerpoint?

Affirmative.

That's odd... the center point of my 5DII is just about as good as other models I use (7D, 50D, the latter is virtually identical to 40D). It's the peripheral points and AI Servo tracking where the 5DII tends to be a bit more disappointing. Center point only and One Shot with stationary subjects, not much difference.

Are you using One Shot?

Is the camera fitted with anything other than the standard or one of the Canon accessory focus screens? I'm wondering if one of the third party focus screens might effect AF in some way.

Do you have the camera set with Assist Points enabled? It's Custom Function III - 7 that enables or disables the 6 hidden Assist Points. They only work in AI Servo, though. I leave them off most of the time.

24-70 is a quick focusing lens, too. You don't have a filter on it, do you? Sometimes filters can mess with AF. The f-stop selected shouldn't matter. Unless you do depth of field preview while trying to focus, the lens is wide open until the very instant of exposure, when it stops down momentarily.

Agree with Alan, use center point with expansion points turned on and go to one shot mode. That is the only way to get the best possible AF performance with the 5d2 in very low light situations.

Expansion points don't work in one shot mode.

Center point is quite good to work in very low light condition. I would try your 50mm f1.4 or the 70-200mm f2.8 to see if problem with your 24-70 lens or something else.

Turn on some more light. If you're shooting at f11 they wont affect the exposure anyways.

Actually I just realized today that its really only a problem when shooting with the Yongnuo YN-128 wireless shutter release.

I have my 5D on a tripod and when I shoot using the viewfinder it acquires AF very quickly every time. When I intentionally move it way out of focus manually, it is quick to obtain AF with just the modeling lamp when I press the shutter release button.

However, when I press the trigger on the YN-128 and look through the viewfinder I see that red AF point flickers repeatedly (as I continue to hold the button) and I can hear the lens trying to achieve AF. Some times it will get it and sometimes it will not. But it requires me to hold down the trigger for about 2-3 seconds sometimes before AF is achieved. At times, AF is not achieved and the camera stops trying even though I'm still holding the trigger button.

That being said, what is so different about pressing the trigger button on the YN-128 vs. pressing the shutter release button on the body? I'm perplexed here.

when you use the wireless trigger, are you in live view mode ?

lannes wrote in post #15447107
when you use the wireless trigger, are you in live view mode ?

When using the wireless trigger it happens both in live view mode and when looking through the viewfinder.

Just turn the lights on.

So you doing remote AF also through your trigger? Wish it was mentioned earlier.

The problem is simply not enough light. I have the same problems. It it can't find af it won't shoot. You can try setting manual focus on a tripod with a wide enough aperture. I often get a small daylight lamp aim it at my model to get af and then aim away to shoot.

All I can tell is to get better lights, like studio strobes, or don't work in low light, or find someway to lock down focus while the lights are on and then turn them off to shoot or some other alternative.

mike_311 wrote in post #15448885
The problem is simply not enough light. I have the same problems. It it can't find af it won't shoot. You can try setting manual focus on a tripod with a wide enough aperture. I often get a small daylight lamp aim it at my model to get af and then aim away to shoot.

All I can tell is to get better lights, like studio strobes, or don't work in low light, or find someway to lock down focus while the lights are on and then turn them off to shoot or some other alternative.

OP problem is not low light and read he is using studio strobes with modelling lights. He is using a radio trigger to activate AF and then take the shot. Problem is trigger is not doing AF. It is not low light, nor camera, nor lens issue.

Do you have your camera set for back button focus? If so enable focusing with the shutter button again.