I'm not a great lens tester - I'm a little weak on the math involved, and a little lazy when it comes to establishing and following a rigorous scientific method. Most of the time I would rather use the lens as intended and see if it works for me or not. But I still like to fiddle with the USAF chart once in awhile, especially to confirm a really good or really bad lens, or to help sort out the sweet spot of a lens.
But I have some questions. First - where can I get a higher resolution chart, rather one that has been printed better, at a reasonable price? I only have the pdf version, printed on my inkjet at (I think) 300 dpi. Most of my lens tests resolve the tiny Group 2 line pairs, partly due to very long focal lengths at my standard 4 or 5 meter test subject distance. But nothing smaller than Group 2 is resolved on my printed chart, and even the Group 2 lines are a little ragged and unevenly separated when viewed under 12x magnification. Second - when using Rob's spreadsheet (USAF lens target to lpmm.xls), does one enter the actual focal length or the actual focal length times the crop factor? 300/2.8 times the 1.5x *istD crop factor makes a big difference in the lpmm numbers; using the 450mm instead of 300mm significantly reduces the lpmm number. Finally - at what resolution do lenses start to look better than average? Much better than average? Much worse than average? -- John Mustarde www.photolin.com
