Often when I print or scan my negatives which I have developed myself, the edges of the picture nearest to the sprocket holes appear lighter in the print than the rest of the image. At first I thought this might be a problem with some out-of-date film stocks, but I am noticing it happen also with fresh stock. Also b&w as well as color. Any ideas what this could be attributable to? I was wondering if over-agitation while developing could do it. The sprocket holes themselves are clear so it seems to a darkening only of part of the image area. I use a plastic tank and wonder if stainless would be better. I agitate by rotating the spiral via the stick thingy that goes into the center. Typically for C-41 I have been agitating by turning the spiral slowly but more or less continuously. Any ideas?