Re: "I reckon that a good 35mm camera (good meaning a top-end lens and
fine grain film) is equivalent to 12-20 megapixels."
My understanding from the camera companies, which I've heard more than once
(and going back to 2000, before any such digital cameras existed) is that
they consider 35mm film about the equivalent of 6 megapixels. It's not
exact, as it depends on what image sensor (not all image sensors of the same
megapixels are the same) and what film (not all 35mm films are the same,
either). The lens obviously enters into overall image quality in a big way,
but that is equally true for both film and digital cameras. But bottom
line, as a rule-of-thumb, and other things (in particular the lens) being
equal or at least comparable, I consider (and a lot of experts consider)
that about a 6 megapixel camera is the equivalent of 35mm film.