Sometimes discussions arise comparing film with digital. It is a bit complex issue. Back in the late 2008 I would have said:
Checking Modern Photography's lens tests we could say that an excellent lens resolves 50 lp/mm on film. 50 lp that is 100 pixels per mm. So a digital sensor would need to resolve 3600 x 2400 = 8.6 MP to match film. A common view was that the real number was closer to 6 MP.
When the first CMOS sensor from canon arrived there was a bit of astonishment, in that they produced very fine results. The images didn't have the resolution but they were very smooth.
A factor to keep in mind is that resolution is not the main factor in the perception of image sharpness. All research indicates that it is the transferred contrast at low resolution that dominates perception.
Blueray film has just two megapixels but it is perceived sharp and a movie in a theatre is shown at even lower image quality. But as long as 10 lp/mm - 20 lp/mm is clean and sharp human vision is happy. The range of resolution needed obviously depends on image size and viewing distance.
But, the above explains a lot, even a 3MP digital camera had enough resolution to fill a movie screen. Sure, it could lack detail on close view, but low frequency detail was good.
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