5D Mark iii constantly hunting for focus

[jaandrews98]

Bought this camera in August, and it's been wonderful.  No problems at all.

The other night during a shoot the camera started to hunt for focus and all of a sudden would not let me shoot b/c it could not find focus.   Worked with it again later and it would be hit or miss, but eventually the more I shot, the more hunting it would do.

Today, 95% of the time it's hunting for focus and not able to find it.  If it does find focus - very seldom - chances are the image isn't sharp.

Here's what I've done:

- Cleared all my settings on my camera.

- Tried other lenses on this camera (same problem -- all lenses hunting for focus)

- Tried the same lenses on my original 5D back-up camera and my 5D is working fine.  No hunting happening.

- I have cleaned the lens and camera electrical contacts.

- I have not dropped the camera, and there's been no off-the-wall damage to it.

Any other ideas? I'm so frustrated and need it to work.

Thanks in advance.

[RexGig]

Are you in single-shot AF mode? If you are in AI Servo, or another AI mode, that your camera may have, the camera is "expecting" subjects to be moving, so will be constantly hunting, or seeming to hunt, for focus when presented with still subjects.

[jaandrews98]

I've tried shooting in both modes.

[RexGig]

In that case, I will defer to others who may have experience with the problem. I have been fortunate with my Canon bodies, with only one having an issue that may yet require Canon service. I have, however, managed to flub some settings, including being in the wrong AF mode for the type of subject, or adjusting FEC when I meant to be adjusting something else.

Well, anyway, welcome to the forum!

[TCampbell]

What focus mode are you using (spot, single, expanded, surround, zone, or full 61-point auto-select?)

Also have you tried moving the focus point or zone?

The sensors are on the floor of the camera.  Behind the reflex mirror which bounces light up to the pentaprism, there's a secondary mirror (behind the first) that bounces light DOWN to the focus points in the floor of that chamber.  If the mirror is sticking (not directing light downward correctly) and or if there's any dust bunnies inside the chamber blocking the focus sensors, then that could cause such a problem.

You can try different focus points and modes.  Remove the lens, put the camera into self-cleaning mode (so the mirror swings up) and inspect for dust.  Unoftunately you wont be able to tell if the secondary mirror is swinging down into the correct position (it would have to go in for service to check that.)