Stopping-down and Focus Shift?

I know very little about what is inside a lens and how the various mechanisms interact, so for those of you who do know, I have a question:

Could a Canon zoom lens have a fault with it that would cause the focus to shift as the camera stops the lens down when the photograph is taken?

I have experienced some unexpected (and intermittent) focus issues over the last eight months which persist, despite me eliminating every possible cause from the photographer and set-up point of view.

However unlikely this is to be the case, and even if it has never previously happened to anyone in the history of the world, if it is indeed possible for it to occur I would be interested to know. I specialize in experiencing problems that no one has ever heard of, so I wouldn't want the fact that it has never happened before to come in the way of someone telling me it could happen.

Thanks

Mike

I guess anything is possible.

Does this happen with all lenses or only one in particular?
Which lens(es) does it happen with?
What evidence leads you to suspect it is being caused by the movement of the aperture?
Has this problem ever occurred when shooting with the lens at its widest setting? If so then you can probably rule out aperture movement as a cause.
Do you focus then recompose?
Have you tried disabling the AF to rule out the camera refocusing when the shutter button is pressed?

Sorry I forgot to mention, it's only with my 17-40mm. My other lenses are spot-on.

What leads me to think up my question is that I'm still getting photographs out of focus when using a tripod, mirror lock-up, remote shutter release, manual focus and f/8. No focusing and recomposing or anything like that. The out of focus parts are always easily within the depth of field.

I know damned well that they are in focus on the screen just before I take the photograph, so the only other thing I could think of was if something was shifting when the lens is stopped down by the camera at the point of taking the photograph.

Another thing that got me thinking was when using auto-focus recently and it set infinity focus way past where it usually does, just for a few photos. The photographs were in focus, but the setting was very strange. I'm wondering if something inside is loose or getting stuck.

Mike

Loose is always possible. If it's loose enough for the aperture movement to throw it off then I'd expect it to be sensitive to just about anything. Does it seem more or less common at different aperture settings? Can you leave the aperture wide open and have no issues? Does the extra movement of stopping down to 16 or 22 make it worse?

I suppose I'll have to test it further because it's my landscape lens and I'm nearly always using it at f/8. Stopping right down is going to be adding other uncertainties with refraction, but I'll see what it does there too.

I'm pretty sure something's amiss here though!

Mike

Mike-DT6 wrote in post #6091661
I'm pretty sure something's amiss here though!

Certainly does sound like something is wrong. Just trying to help pin-down the problem.

Yes, I appreciate that, thanks. Maybe I should hit it a few more times.

Mike

Mike-DT6 wrote in post #6091680
Maybe I should hit it a few more times.

The Bigger Hammer approach always works well.

Don't tempt me!