Wide angle conversion lens?

I have both Nikon D5100 and Nikon D60 and using both 18-55mm lens and other telephoto/macro lens with.
I have gone through few articles and reviews, just few question: - on some wide angle conversion lens, when we focus or zoom, the side edges of the conversion lens appears on the pic with a dark spot on the corners, but not with all conversion lens.
There are few on markets, for example, some are 52mm, 58mm. 60mm
what difference does it make if attached to a nikon 18-55mm lens with any of the above conversion lens?

any help or ideas which one to buy?
thanks

Added (1). How do i make sure i have a wider range diameter of the conversion lens in order to avoid the dark edeges? Any spec or guide i have to follow?

If the conversion lens is less than the camera lens, vignetting will occur, which is the darkness at the corners. By all means, play with the conversion lens toys. But if you get a real taste for wide angle, look at the Tokina 11-16mm f/2.8.

Typically you'd want to get the one that has the same thread size as your lens. If the conversion lens has a very low multiplier like.4x, or if the overall length of the lens is too long, then you'll start to see the edges of the lens as dark or black edges on the picture.