I've noticed recently that the corners of the frames of my XL1s's images frames are slightly darker than the rest of my image, with the effect getting worse at the telephoto end of a zoom. This is with either lens (IS II or the manual 16x). It sort of looks like vignetting, but I don't see how that could be. I don't remember seeing it before. When zoomed fully in, it looks like a I have some sort of tube pressed up to the lens, and I'm looking through it. Some weird function of the iris? It seems more noticable when fully open, but I'm doing these tests inside under not very good lighting. Do your cameras all do this?

Photo-optical lenses have some fall-off at the corners. Check the lens reviews in photo magazines like Popular Photography for some quantitative data. Best way to demonstrate it it is with flat shubects (e.g., a white wall) that are somewhat underexposed.

Are you shooting with something attached to the front of the lens (filters, hoods) that you didn't in the past attach? If so, the filter etc. is causing vignetting. The edges of the filter are intruding into the frame and the result is dark corners. Other wise you're observing, as Don suggests, fall off of the light. This is most common at the wide angle position on lenses.

Josh I get the same thing when I'm using a filter, but not all the time.

No filter, yes, using the lens hood. Maybe I've just never noticed before. I can make some screen grabs if someone wants.

Johs, you probably see "portholing". Do a search in this forum and you will find some answers.

If you want to find out what it is you'll need to make some grabs with different aperture settings. As you stop the lens down (go from F/2.0 to F/16 or higher) you are increasing DOF and the likeliness of the outer edges of a filter or hood protruding into the scene. The wider the angle of view the greater the chance of light fall off and vignetting. So, run some tests and keep track of F/ Number, lens to subject distance, and angle of view.

Josh,
Your observation sounds very similar to a phenomenon I observed on my GL2 (http://www.dvinfo.net/conf/showthread.php?s=&threadid=3430&highlight=keyhole) last summer. I've never observed it on my XL1s. After lengthy discussions and further study I am convinced that, as Jeff and Don suggested, it's an optical characteristic of the lenses.

I'm guessing you're right. . .but I've never noticed it before. I guess the corners are outside the safe area, so they only show up on a computer monitor when editing, not when played on a TV.