I just did a 20 x 24 from a color 4x5 of some flowers and I noticed in the corners, the image got soft. I used a polarizing filter. It is sharp every where but the corners. it is not dim in the corners. It is from a 75mm F5.6 Caltar II N (80s Grandagon by the Serial Number and the info on View Camera Magazine.) I am under the impression that this is as good a short lens as there is. I used a slight forward tilt, was focused at hyperfocal and was at F32 (pushing difraction here?) I suppose it is possible that the filter could cause it. It is a good double coated filter. I was just wondering if anyone else had this kind of experience with their 75 mm lens and at what aperature do you consider the limit (either way)