Reply to post: "35mm film is equivalent to about 8.6MP"

"35mm film is equivalent to about 8.6MP"

Depends what you mean by "equivalent".

Disregarding the lens for now (although it's often the limiting factor for both film and digital photography), a monochrome film may have anything from 1M to 3bn grains per cm² (fast v. ultra fine grain film), the lower figure nominally approximating to your 8.6MP, but that's the very bottom end of the scale for film. Colour films tend to the lower end of this range and the dye substitution process blurs the grain a bit, but that can actually increase subjective image quality as it tends to eliminate jaggy edges.

However, on a film the grains are randomly distributed and vary in size in a range of some 200:1, whereas the pixels of a digital array are on a rigid rectangular grid. This alone makes the film better able to record lines and edges that are not strictly perpendicular to the frame, as jaggy digital diagonals are unavoidable unless smooted artificially by post processing.

Then you have the effect of the Bayer filter, which averages a sliding array of (minimum) 4x4 pixels in order to render colour at all. So despite a lot of fancy math, you can roughly divide the linear pixel count by four to arrive at the actual resolution of a digital camera image. Furthermore the actual pixels out do not directly represent the image that landed on the sensor as they are the result of averaging, smoothing and various "enhancement" algorithms.

Consequently, given the same lens, a film image is a nearer approximation to what was being photographed than a digital image can ever be, and even with clever post processing you should probably multiply the "equivalent" pixel count by at least a factor of three. Funnily enough, for a 35mm frame that's around a 24-30 MP camera.