5DII & 24-70

[soboyle]

My 24-70 on the 5DII seemed a bit flakey on my last shoot. Either the camera or the lens kept resetting the lens to a wide open aperture from my selected aperture. I'd be shooting at f/8 for a few frames, then notice that I was wide open again.
I'll send it in for repair, but wondered if others had seen symptoms like this before, and what the problem might be. I suspect it is the lens, maybe just the connected between lens and camera. Other lenses seem OK.

[sbay]

I had the the aperture blades stuck wide open on a 70-200 lens before. Resulted in the pictures being totally overexposed.

[john2]

I have had this problem with a couple of Nikon lenses. The problem was that the aperture blades were sticking to each other - a quite expensive service was required.

[JohnTodd]

A mistake I've made with the 5D and 5D2, particularly in studio shooting, is registering an aperture and shutter speed to C1-C3, then changing the aperture to account for lighting changes. If the camera goes to sleep, when you wake it up, it's reverted to the original, stored aperture setting. (Which is completely correct - it's operator error not to check it.) Could that explain what you're seeing?

[soboyle]

I checked the aperture blades using the depth of field preview button, and they seems to be working fine at all apertures. It seems to be an intermittent problem, so it may be a camera/lens contact issue.