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A telecentric lens is a special optical lens (often an objective lens or a camera lens) that has its entrance or exit pupil, or both, at infinity. The size of images produced by a telecentric lens is insensitive to either the distance between an object being imaged and the lens, or the distance between the image plane and the lens, or both, and such an optical property is called telecentricity. Telecentric lenses are used for precision optical two-dimensional measurements, reproduction (e.g., photolithography), and other applications that are sensitive to the image magnification or the angle of incidence of light.
The simplest way to make a lens telecentric is to put the aperture stop at one of the lens's focal points. This allows only rays including the chief rays (light rays that pass through the center of the aperture stop), that will be about parallel to the optical axis on the other side of the lens, to pass the optical system for any object point in the field of view. Commercially available telecentric lenses are often compound lenses that include multiple lens elements, for improved optical performance. Telecentricity is not a property of the lenses inside the compound lens but is established by the location of the aperture stop in the lens. The aperture stop selects the rays that are passed through the lens and this specific selection is what makes a lens telecentric.
If a lens is not telecentric, it is either entocentric or hypercentric. Common lenses are usually entocentric. In particular, a single lens without a separate aperture stop is entocentric. For such a lens the chief ray originating at any point off of the optical axis is never parallel to the optical axis, neither in front of nor behind the lens. A non-telecentric lens exhibits varying magnification for objects at different distances from the lens. An entocentric lens has a smaller magnification for objects farther away; objects of the same size appear smaller the farther they are away. A hypercentric lens produces larger images the farther the object is away.
A telecentric lens can be object-space telecentric, image-space telecentric, or bi-telecentric (also double-telecentric). In an object-space telecentric lens the image size does not change with the object distance, and in an image-space telecentric lens the image size does not change with the image-side distance from the lens.