Scan resolution

Tom Ang (Digital Photography An Introduction DK) gives the following formula:

input resolution = (size of output x output resolution)/size of orignal

Suppose your original is 2 inches long and you want a print 10 inches. If you need an output resolution of 300 lpi (lines per inch) then you need

10 x 300 pixels

and the input resolution must be

3000/2 = 1500 ppi (points per inch)

The important thing is to know what the output resolution may be

• inkjets between 80 and 200 lpi
• screen 72 dpi or 96 dpi

He also gives some rules of thumb based on file size.