Can anyone tell me how to size up the performance of a lens for myself, as oppos ed to trusting what someone else thinks is good. Specifically, (on a 4X5) I have an Ilex Paragon Anastigmat F 4.5 E.F. 5" Series S on an Ilex optical co. #1 uni versal shutter: tested out as 1 3/4 1/5 1/10 1/25 1/50, or as the guy who tested it for me said, this is a good one. F ranges from 4.5 to 45. Glass is uncoated. Has what appears to be one rear element and two front elements, although I coul dn't unscrew the front elements to see hwo they were made. Subjectively, it seem s to me that for some reason the lens is sharp and performs better at f 8 than a t f 16 or 22. In theory this shouldn't be the case. Then I read that a tessar de sign, which this seems to me to be, shouldn't be used but under f 11. Sometimes this old lens seems sharp and at other times it seems really bad. How do you det ermine what's what with a test. Or how do you make optimal use of a half-way dec ent lens? Also, how can you test a lens for circle of illumination? Would shutte r vibration be a factor, and how would you know you had it if it was a problem? Or what used lenses are famous for their tonality range? Thank you, David