There is no one answer, because film doesn't have to bother with pixels.
Here are the facts so you can decide.
The best comparison I've seen is ISO 100 film compared to 10MP.
With film, the image is continuous in all three dimensions: x, y, and z (intensity).
With film, you get the same resolution at color transitions (green/magenta, for instance) as you get for light/dark transitions.
With film, you have complete R, G, and B resolution at every point.
Film's sharpness decreases gradually as the pitch (spatial frequency of fineness of detail) increases.
Seen as an MTF curve, film's response to detail gradually becomes less as the details get finer.
Film can resolve insanely fine details, but not with as much contrast as coarser features.
This natural response is similar to our eyes, and another reason film looks so good.
Digital Film Scans
When you scan film, good scanners resolve right up to their DPI (dots or pixels per inch) rating.
Film scans also have complete RGB color information and resolution at each pixel.
Film scans resolve detail about as well as the original film, up to the resolution of the scanner. There is no response to details finer than the resolution of the scanner, even if it's on the film and visible in optical prints.
When laypeople compare film to digital, they aren't comparing film to digital. They are usually only comparing scans of film to digital.
With digital cameras, you get full contrast up to the very highest limit of the sensor's resolution. Finer details simply disappear, or become aliases.
This is one way film and digital look so different. Film records fine and coarse details naturally, while digital (and video) tend to record medium details more strongly than film, but have no response to the extremely fine details which film can record.
Often the finest medium details to which the digital camera is sensitive are boosted in contrast. This is called sharpening, and is how we get digital images to fool the eye into thinking they're sharp.
Digital cameras never resolve their rated resolution. The only digital cameras that used to were those with Foveon sensors, but them Sigma started lying, too.
All (non-Foveon) digital cameras use a black-and-white sensor on which red, green and blue dots have been painted. Since we only have about a third the resolution in any one color, since only one-third of the sensor is painted with each color, Bayer Interpolation firmware in the camera (or in raw conversion software) takes the pixels of each color, and interpolates (smoothes) values in-between the pixel locations of each color to create brightness value for each color at every other color's location.
Therefore, at each pixel location in a digital camera's image, we don't have full R, G, and B data. We only get about half, which is why digital camera images at 100% won't look as good as good film scans at 100%, or lower resolution settings of your camera seen at 100%.
This is all called Bayer Interpolation. With this, most digital cameras really only resolve about half their rated megapixel rating. For instance, a 10MP camera really only sees about as well as a theoretically perfect 5MP digital camera, or 5MP film scan.
Foveon chips see at full resolution, but the makers of those cameras lie about the resolution to keep up with other cameras. Most Foveon-chipped cameras (Sigma) multiply the real resolution by three! What Sigma sells as 14MP cameras are really only 5MP.
The Digital Resolution of Film
So how many pixels does it take to describe all the detail we can get from film?
Each line will require one light and one dark pixel, or two pixels. Thus it will take about 320 pixels per millimeter to represent what's on Velvia 50.
320 pixels x 320 pixels is 0.1MP per square millimeter.
35mm film is 24 x 36mm, or 864 square millimeters.
To scan most of the detail on a 35mm photo, you'll need about 864 x 0.1, or 87 Megapixels.
But wait: each film pixel represents true R, G and B data, not the softer Bayer interpolated data from digital camera sensors. A single-chip 87 MP digital camera still couldn't see details as fine as a piece of 35mm film.
Since the lie factor factor from digital cameras is about two, you'd need a digital camera of about 87 x 2 = 175 MP to see every last detail that makes onto film.
That's just 35mm film. Pros don't shoot 35mm, they usually shoot 2-1/4" or 4x5."
At the same rates, 2-1/4" (56mm square) would be 313 MP, and 4x5" (95x120mm) would be 95 x 120 = 11,400 square millimeters = 1,140 MP, with no Bayer Interpolation. A digital camera with Bayer Interpolation would need to be rated at better than 2 gigapixels to see things that can be seen on a sheet of 4x5" film.
As we've seen, film can store far more detail than any digital capture system.
The gotchas with any of these systems is that:
1.) It takes one heck of a lens to be able to resolve this well.
2.) It takes even more of a photographer to be able to get that much detail on the film, and
3.) If you want to scan the film and retain this detail, you need one hack of a scanner (320 lpmm = 8,000 DPI).
This is why every time higher-resolution film scanners came out back before amateurs could afford DSLRs, we saw more details where we though we wouldn't see any.
Consumer 35mm scanners hit 5,400 DPI (Minolta) before the amateurs went to DSLRs, and even at 5,400 DPI we still saw more detail in our scans than we did at 4,800 DPI.
Film never stopped amazing us as we scanned it higher, and this is why.
5,400 DPI is equal to 212 pixels per mm, or 0.045MP/mm^2. Thus a 35mm slide, scanned on that Minolta 5400 scanner, yielded 39MP images, without Bayer Interpolation. Open these in PhotoShop, and 39x3 = 120 MB files, again, sharper than the Bayer-interpolated images from digital cameras.
Resolution has nothing to do with getting the right pixels and making a good photo, but if all you want to do is count pixels, count on film. See also Why We Love Film.
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