Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 106

Color Depth

Line art, text
(one color,
bitmap)

Grayscale (BW
photo)

Special GIF
(indexed
colors)

Color (color
photo)

Color depth 1 bit 8 bits 8 bits 24 bits

(true color)

Second power 2 1 2 8 2 8 2 8 x 2 8 x 2 8

Number of
color values

2 256 256 approx. 16.78

million

Selecting a Resolution

Let's review resolution because it is tremendously important. The resolution
determines the number of image dots (pixels) per length unit (inch or
centimeter) in any given image. The screen resolution is normally measured
in dots per inch (dpi). In the printing industry, the resolution of the printing
screen is usually calculated on the basis of the number of lines per inch.
When scanning original documents on a flatbed scanner for printing
purposes, you should select the highest possible resolution. If the image is to
be printed in its original size, 300 dpi is a good standard.
When scanning images for publication on the Web, it is a good idea to
initially select a higher resolution for editing purposes. A high resolution will
keep the sharpness and contrast of the image high while you edit, even if you'll
eventually be reducing the resolution. Remember to reduce the resolution on
a copy of the original image because the process cannot be reversed.

Formula for the Scanning Resolution

When you scan original materials, you can calculate a scanning resolution for
your print output. (Professional scanners achieve a much higher resolution
than flatbed scanners manufactured for home use.)

Resolution (desired) × scaling factor × scan factor =
scanning resolution

A scan factor between 1.4 and 2.0, inclusive, will normally provide a good
result.