A telecentric lens is a compound lens that has its entrance or exit pupil at infinity; in the prior case, this produces an orthographic view of the subject. This means that the chief rays (oblique rays that pass through the center of the aperture stop) are parallel to the optical axis in front of or behind the system, respectively. The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points.
An entrance pupil at infinity makes the lens object-space telecentric. Such lenses are used in machine vision systems because image magnification is independent of the object's distance or position in the field of view.
An exit pupil at infinity makes the lens image-space telecentric. Such lenses are used with image sensors that do not tolerate a wide range of angles of incidence. For example, a three-CCD color beamsplitter prism assembly works best with a telecentric lens, and many digital image sensors have a minimum of color crosstalk and shading problems when used with telecentric lenses.
If both pupils are at infinity, the lens is double telecentric (or bi-telecentric).