AF problems with the 70-200 F4 is?

I've noticed in the studio that this lens seems to have trouble focusing. I was using f 6.3, ss125, iso 100 with my strobes. I also have CFn 13 set to 1, I use the back button focusing. I had my modeling lights on about 3/4 power. Anyone else have this problem and what did you do about it? The little red focusing point never came on. I even tried taking the CFN back to default, no go. I haven't used it that much, but recently started doing portraits in the studio with it and love the results. Could it just be too dark in the studio for it to find focus and if so, how to fix that?

Was it focus hunting? Were you able to focus afterwards with another lens?

Are you in AI Servo?

If it's really dark in the studio, look into buying the 580EXII and using its AF-Assist light. It should allow you to focus in darkness.

yup, AI Servo. I suppose the 430ex does not have an af-assist light? I couldn't hear it trying to focus, seemed dead. However, somehow, most of the pix were in focus.

BeckyMax wrote in post #6189771
yup, AI Servo.

That's why, mine does the same thing. switch back to AI Focus or Oneshot.

(1)AF assist light will NOT fire in AI Servo
(2)I'm pretty sure the 430 has the AF assist light, but it only works in One Shot
(3)If you're in AI Servo, I don't think anything beeps or lights up (since it's always re-focusing)
(4)You couldn't hear anything because USM is "silent"