I am used to do multiple scans of the same picture because "this reduces noise." Tired of waiting to get my scans done I decided today to run an experiment. Setup: Epson 4870 with Silverfast Ai. Scanning positive film with 1, 2, 4, 8, 16 scans. No ICE, 3200dpi.

Result: the noise is indeed progressively reduced from 1 to 16 passes and one can see that on the screen at magnifications above 100%. However, the reduction in noise goes together with a reduction in sharpness. I was able to take the picture scanned once and reduce its noise at the same level of that scanned 16 times by applying in Photoshop a gaussian blur with a radius of 0.7 to it. (Of course I also lost some sharpness in doing so.)

Bottom line: it seems that scanning a film once and applying a tiny bit of blur produces the same result of scanning the film 16 times. Or, to put it another way, that scanning the film more than once buys you nothing: you might be better off in fact simply scanning it once and applying some selective blurring to high noise areas.

What am I missing here? Any word of wisdom?