High scan resolution is important only in line art scans. It should be between 800 to 1000 dpi depending on the printer resolution.

The best possible scanning resolution is very important and should be well thought out "the more the better" does not always hold true!

To clarify:
Shades of grayscale images are converted into dots made of a 16x16 matrix when imaged on to an image setter. Ideally, a screening dot contains 256 individual pixels. When a photograph is scanned and printed onto a 150 line screen, each grayscale dot is converted into a 16x16 matrix. An image setter with a resolution of 2540 dpi is just capable of imaging such screening dots.

Since there are losses in the process of analog-digital conversion, an additional Q-Factor (Q for Quality) is introduced. This factor is generally 1.4, and, with exception, 2.0.

As a result, the following formula for the calculation of the ideal scan resolution may be used:


For example: The scan resolution for a 150 line screen with a 1:1 scaling-factor has to be calculated.

Scan resolution = 150 x 1.5 x 1 = 225 dpi

The resulting file size for an A4-page for black and white gray-scale is 5.77 MB, for color 17.3 MB. 300 dpi would roughly double the file size. This shows the importance of setting the optimal resolution since memory requirements and processing time increase dramatically.

For a 121 line screen for a newspaper and a scale-factor of 50% the calculation is as follows:

Scan resolution = 122 x 1.5 x 0.5 = 91,5 dpi

If you want to double the image size:

Scan resolution =1 22 dpi x 1,5 x 2 = 366 dpi.