You can calculate the scanning resolution you require by measuring the original picture. Because resolution is normally expressed in inches, calculations are easiest when the dimensions of the original are in inches as well.

A 35mm film frame measures 1.5 x 1.0 inches. If you want to print the scan at snapshot size, the scanning resolution should be 300 ppi x 1.5 x 6 inches, which is 2700 dpi. (Because pixels are square, it’s only necessary to make this calculation for one side of the image.)


Epson Perfection V800 Photo

To print a scan from a 35mm frame at A4 size (11.7 x 8.3 inches), the calculation is 300 x 1.5 x 11.7 = 5265 dpi. For A3 size (16.5 x 11.7 inches), the scanning resolution must be 7425 dpi, while an A2 enlargement (23.4 x 16.5 inches) requires a scanning resolution of 10,530 dpi.

Scanning at a higher-than-necessary resolution is time-consuming and produces more image data than you need. However, the digitised image can be printed at smaller output sizes without loss of quality.

Image quality is always sacrificed when you exceed the maximum output size for the image resolution. Scanning a 6 x 4 inch snapshot print at 1440 dpi produces an image with 8640 x 5760 pixels, which could be printed at A2 size, although the end result may contain artefacts because of the high degree of enlargement. It would be better to scan at 600 dpi, which produces a 3600 x 2400 pixel image. This allows you to enlarge the image to around A4 size.


Excerpt from Mastering Digital Photography 3rd Edition.