If you get an out of focus band across prints from a roll film camera, film flatness is the prime suspect. The first poster in this thread reported something rather different to this.(posted 8225 days ago)For some excellent articles about diffraction and film flatness, go to the Zeiss site, >photography >articles on lenses >archives.
This might be a good shortcut if it comes out on one line:
http://www.zeiss.de/de/photo/home_e.nsf/Inhalt- FrameDHTML/4FDEACEDCB7D0AF541256A53003923AA