What Cause Black Shadows at the Corner of photos / Vignetting

Examples of Vignetting 

http://www.photographymad.com/files/images/beach-vignette.jpg

http://cdn.picturecorrect.com/wp-content/uploads/2010/12/vignette-type2.jpg

http://nenuno.co.uk/creative/wp-content/uploads/2010/07/vignetting.jpg

Problems and Solutions:

This is the most simple and common cause, also called vignetting, it can also be caused by an inappropriately sized lens hood or filters - especially when more than one filter is stacked. The extended filter rings or the hood block the lens's view at the corners.

Most likely an incorrectly oriented lens hood. Lens hoods for zoom lenses are petal shaped, with cutouts which are projected to avoid shading the corners. The largest pieces 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.

Some hoods have two long flanges and two short ones. If yours does, then try placing the long flanges on the top/bottom side of the lens rather than the left/right. Because of the native aspect ration of your film plane (chip, sensor, pick a name) (i.e., it is a horizontal rectangle) the long flanges will appear at the left and right of the frame rather than the top and bottom. This is because there is less sensor to "see" the hood on the top and bottom.

There are few other factors too.

1. Your Flash coverage narrower than the lens's field of view. In this case it can be avoided by using indirect bounce flash.
2. The polarizing filter is too big for the wide angle setting of the lens, so it "hides" the light in the corners.
3. Maybe you can find a slimline polarizing filter which has better results.
4. There is Something in the direction of the light.
5. You are using a small aperture and longer exposure.
6. You may have a lens hood that is too narrow for the lens, or a filter rim that is too thick. A polarizing filter, for example, often has a higher ring than other filters.
7. 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.
8. You may have the wrong hood for that lens. A shorter hood may work better for you.It could be you're using a lens designed for a smaller sensoand then so the image circle doesn't illuminate a large enough area, leaving dark corners.
9. Finally, there could be something loose inside the lens which is causing mechanical vignetting.

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