What size film?
Film, like digital files, comes in many resolutions. 35mm is an amateur format, medium format (120 or 6×7) is for head shots, and large format (4×5″ and up) is for landscapes.
Arizona Highways prefers 4×5″ film. As of 2008, they now accept digital images, but with a catch: they have to be at least 300 DPI at 12×18,” or 20 MP. They say an 8MP camera is OK, but you’ll notice that you have to supply 12×18″ at 300 DPI, which is 3,600 x 5,400 pixels, or 20 MP.
If you do fret the pixel counts, I find that it takes about 25 megapixels to simulate 35mm film, which is still far more than any practical digital camera. At the 6 megapixel level digital gives about the same sharpness as a duplicate slide, which is plenty for most things. Honestly, I have actually had digital files written back out onto film to see this. See also my film vs. digital page here.
Of course I use much bigger film than 35mm for all the pretty pictures you see at my website, so digital would need about 100 megapixels to simulate medium format film, or 500 megapixels to simulate 4×5″ film. This is all invisible at Internet resolutions, but obvious in gallery-size prints.
Taken from Ken Rockwell