Just wanted to check if anybody has experienced the below problem before. I purchased the lens 2nd hand but the seller denies any knowledge of the problem (yeah, right!). I suspect a trip to Canon is in order for this lens but thought I'd run it by everyone here first in case there is some simple solution I've overlooked like shaking it vigorously or perhaps performing a rain dance.
My 24-105 f/4L is having problems with the aperture behaving erratically. When the camera is set to any aperture other than f/4, the iris will constantly change between several different settings. When zooming in, it gets darker (ie the lens doesn't compensate by widening the iris at longer focal lengths as it should to maintain the same aperture ratio. I'm not talking about a flicker, I'm talking about an obvious ~2-stop darkening of the image through the viewfinder) but when zooming out again it stays dark still.
The aperture setting in the camera is not reflective of what the lens iris is actually set to. Eg: the camera might read f/4 while the lens is actually at f/22. Changing the aperture does sometimes cause the iris to adjust, but only within the range of the camera - ie: if the camera reads f/5.6 but the lens is actually at f/22, I can only open up by one stop, because even though the lens is now only at f/16, the camera now thinks it is at it's limit of f/4 and so will not adjust it any further.
And as a precursor to all the usual generic responses, so far I've tried the following: same lens on multiple cameras, different lenses on same cameras, every combination of IS/AF on/off, cleaning the contacts on both the lens and cameras, camera powered by a fully-charged battery, in M mode, live view/movie mode and regular stills, and played around with the DOF preview button. Curiously, the DOF preview does not do anything but when using live-view AF via a half shutter press it does open the iris up to brighten the image, but only temporarlily. And yes, it is definitely the iris and not the shutterspeed/ISO messing up because a) it's in M mode with nothing on auto, and b) because I can see the iris blades flickering erratically through the front of the lens.
Thanks in advance for any help you can give me.... even if that help might be to tell me where my nearest Canon repair centre is!