I have a couple of prints (prints with black backgrounds) (example http://moltenwords.net/picto/index.php?p=42) where the corners are not as dark as the rest and someone mentioned once that it could be light fall off in the enlarger. So I am thinking maybe I should test for that. If I just expose a piece of paper for a while, with no negative, then it should all be even, right? And then if it isn't, it shows I have a problem? But then how do I fix that? Is that an alignment issue?