Rules of sharpness
From The Glossary of Digital Photography
Refers to the perception of sharpness by the viewer, which involves the viewing distance, size of the image, compression of the image, contrast, and other factors. The rules illustrate that scanning an image at a higher resolution will produce a better-quality (and sharper) image than scanning it at a lower resolution. Also, scanning an image at a higher resolution will generally produce a better-quality image than scanning at a lower resolution and then interpolating it up to the size of the higher-resolution image. Finally, users need to be aware of the amount and type of compression applied to images after scanning, as that will affect the perception of sharpness as well.
