Cover the top half of a camera lens. What effect does this have on the pictures taken?

Depends on the field of view of the lens and the size of the lens.

For wide field of view lenses, there are two basic effects. The depth of focus is so close that half the image will be obscured; I've got lots of photos with fingers and thumbs in the way. The second effect is that the image is slightly dimmer, because the collecting aperture has been reduced

For narrow field of view lenses, there are two effects. The depth of focus is a considerable distance away from the lens, so you'll get the entire image, only dimmer. The second effect is that the image will be blurrier, because the reduced aperture also degrades the sharpness of the image.

The "bottom" of your picture will be cut off.

The bottom of your picture will be the only part you see. The top will be dark, with a fuzzy edge halfway down, because the lens will have focused beyond it.

You'll only have the bottom half of a picture.

It would be a creative effect for the picture but it depends on the film that you are going to use. If you are going to use a digital camera the better. I tried doing it once and it is a trick to play on your friends. You do not need to cover the lens with the hand right? You could cover it with cellophane to get a very exciting effect or you could use cut patterns to get ceratin effects. There are many things that you can do with just the idea of covering the top half of your lens, try using them all and make a list of all the effect you've created. Happy Taking!!

Top half becomes black, you only have the bottom half image.

Camera now is straight forward, WYSIWYG.