There are completely black corners in some of my photos. What can cause it?
I have a full-frame Canon 5D Mk II and had the problem with my 24-105 Canon lens recently and have experienced it with my 12-24 Sigma lens before.
Most likely an incorrectly oriented lens hood. Lens hoods for zoom lenses are petal shaped, with cutouts which are designed to avoid shading the corners. The largest bits of the petal should be top and bottom, if the hood is rotated it could be the corners of the image are being blocked by the hood.
It could be you're using a lens designed for a smaller sensor so the image circle doesn't illuminate a large enough area, leaving dark corners. Finally there could be something lose inside the lens which is causing mechanical vignetting.
There are two factors:
Something in the way of the light.
A small aperture.
You may have a lens hood that is too narrow for the lens, or a filter rim that is too thick. A polarising filter for example often has a higher ring than other filters.
The small aperture (high f-stop number) makes the lens hood or filter rim come closer to be in focus, so that it causes a black corner instead of just severe vignetting of the corners.
I experienced the same problem with my 18-270 lens at the low end of the zoom and with a polarising filter. I tried it with and without hood and changing the aperture and shutter speed. What I found was that vignetting occurs whether the hood is there or not, but occurs at the 18mm zoom with the higher shutter speed. With a lower shutter speed or higher aperture it lets more light into the lens and so the dark spots disappeared. Unfortunately if I needed faster shutter speeds (photos of wildlife) the problem reappeared. It was a balancing act and one of the problems in having a telephoto lens of such a large range (or so I understand). I would be glad to hear of other people's solutions.
I experienced this the other day when shooting video (Canon 5D Mach II w/ 16-35mm), and I believe it was due to the Shutter speed as I accidentally turned the wrong wheel. When I noticed that I went below 1/60th of second to about 1/30th and readjusted the vignetting went away. Truth is I did not notice the vignetting until I played the footage back. Fortunately, I noticed the wheel error before recording a multitude of video clips.