The Digital Resolution of Film
So how many pixels does it take to describe all the detail we can get from film?
Fuji Velvia 50 is rated to resolve 160 lines per millimeter. This is the finest level of detail it can resolve, at which point its MTF just about hits zero.
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.
35 mm film is scanned for release on DVD at 1080 or 2000 lines as of 2005.
The actual resolution of 35 mm camera original negatives is the subject of much debate. Measured resolutions of negative film have ranged from 25-200 lp/mm, which equates to a range of 325 lines for 2-perf, to (theoretically) over 2300 lines for 4-perf shot on T-Max 100. Kodak states that 35mm film has the equivalent of 6K resolution according to a Senior Vice President of IMAX.
A 15 perforation 70mm IMAX film negative captures at an estimated 18K, which is the equivalent of 18,000 horizontal pixels.
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.