Calculating theoretical megapixels from scans rellies on a mountain of fallacies. It just doesn't work that way. My black and white 4x5 scans are 56 megapixels (after down-sampling to my printing resolution). And yes, the results look great at the sizes I print. But I know from my recent digital experience that comparable results could be had from fewer than half that number of pixels, if they came from a good sensor.
A lot of this has to do with the MTF characteristics of film, a lot has to do with s/n performance of the film + scanner vs. the digital sensor. I don't know what all the factors are. But I do know that a high quality 80mp sensor would be capable of stupefyingly detailed and sharp results.
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