You guys can look up bellows extension factor- it's well known that as you focus closer, your effective aperture decreases and you require more exposure.
As far as using wide angle lenses goes, one thing you have to worry about is the maximum extension you can get while still being able to focus. As you add extension, the distance to the subject decreases. At some point the location your lens is focused on is actually before the front element of your lens, meaning you can't focus at that magnification. Since the focal length is shorter, this happens sooner with wide angle lenses. Of course, the solution in this case is just to use a reversing ring.