Scan the film image. Observe it on your computer and analyze it using ImageJ or PixelProfile. Works well with 4000 dpi scanners and digital cameras but doesn't quite cut it for measuring lens resolution with 2400 dpi scanners. If you print digitally this technique has the advantage that it accurately represents your imaging system. Using the |sinc|3 approximation for a properly functioning film scanner, the 50% MTF frequency (f50) is about 0.366*dpi (or dpmm), and the 10% MTF frequency (f10) is 0.629*dpi. For a 2400 dpi (94.5 dpmm) scanner, f50 = 34.6 lp/mm and f10 = 59.4 lp/mm. Not impressive. For a 4000 dpi (157.5 dpmm) scanner, f50 = 57.6 lp/mm and f10 = 99 lp/mm. This is good enough to distinguish between good and excellent lenses, but I'd want more if I could get it. Sharpening boosts a scanner's f50 and f10, but it can confuse the results if you don't apply it with consistency.