The offset printing process cannot print different density of ink across an image. To achieve a tonal range the image is broken up into fine dots using a screen, (mechanical or electronic). A finer screen gives a more gradual tonal change, sharper picture and finer reproduction. However when printing fine screens the inter dot spaces may fill with ink, or a rough paper surface leaves unprinted dots.

It is important to set the fineness of screen dependent upon the smoothness of the stock, the printing process used, the quality of the artwork and the effect wanted. Sometimes lower screen rulings are selected to give a "grainy" result.

In four-colour process printing, screen angles are different for each of the colours to reduce a moiré effect from overlapping screens.

Stochastic screening is an electronic screening process where the screen spacing is manipulated to give a randomly sized dot, eliminating any moiré pattern.

High quality coated stocks, (e.g. Novatech) will reproduce work as fine as 200/ 250 lines per inch with conventional offset, up to 300 with waterless. Smooth offsets, (e.g. Look!), are recommended down to 175 line screen while text and cover stocks (including economy uncoated offsets) are recommended to be printed with 133 to 150 line screens.