Well that’s a question. There’s no easy answer, but I’ll discuss it a little here. If you just want the figures, scroll down to the table!
I’ve checked in various places, and the consensus seems to be that a frame of 35mm film (24mm × 36mm in size) has up to 20 megapixels, assuming the film is fine-grained, and was exposed and developed properly. We arrive at this number by scanning a negative at progressively higher resolution (in terms of dots per inch, dpi), seeing smaller and smaller artefacts in the picture, until we reach a stage where increasing the dpi of the scan does not yield more smaller grains in the scan. At this stage, we see what digital resolution we have scanned the image at.
There’s massive scope for mishandling film and reducing the fineness of the grains, which means we will effectively lower the digital resolution of the negative. It’s extremely hard to judge this here, so we will assume that our hypothetical photographer was careful in handling his film. We will also ignore any loss of sharpness caused by imperfections in the lens.
Taking the optimum value of 20 megapixels for a 35mm frame, and extrapolating this to other film formats…
|35mm (135)||24 × 36||20|
|120 6×4.5 medium format||60 × 45||63|
|120 6×9 medium format||60 × 90||125|
|5×4″ large format||127 × 102||300|
|10×8″ large format||254 × 203||1,200|
As I touched on earlier, raw “pixel” (grain) count doesn’t necessarily give better resolution with film. Grains on film are different sizes and not in a neat grid, like pixels in a digital camera’s sensor. True resolution, as seen with the eye, has more to do with resolving closely-spaced pairs of fine lines.
Given that you usually need more film grains than digital pixels to achieve the same optical resolution as seen by the eye, it’s a reasonable assumption that my 35mm SLR (~20 megapixels) is similarly performing to my Canon 450D DSLR (12 megapixels).
Some high-end digital cameras such as the Hasselblad H4D-60 can now achieve 60 megapixels but this still doesn’t come close to professional medium format cameras using 120 film (still used for lots of magazine and glamour photos), or large-format cameras (still almost exclusively used for landscape photography).
So there you have it. Film can produce a considerably higher optical resolution than digital cameras, when used carefully.