Object-space telecentric lenses
An idealized object-space telecentric lens, with one thin positive-powered element, has its aperture one focal length behind that element
Non-telecentric lenses exhibit varying magnification for objects at different distances from the lens. Most lenses are entocentric—objects further away have lower magnification. For pericentric lenses, objects further away have higher magnification. The variation of magnification with distance causes several problems for machine vision and other applications:
The apparent size of objects changes with distance from the camera.
Some features or objects may be hidden by objects that are closer to the lens.
The apparent shape of objects varies with distance from the center of the field of view (FOV). Objects appearing close to the edges are viewed from an angle, while objects near the centre of the FOV are viewed frontally (circles near the centre of the FOV become egg-shaped when moved towards the periphery).
Telecentric lenses, on the other hand, provide an orthographic projection, providing the same magnification at all distances. An object that is too close or too far from the lens may still be out of focus, but the resulting blurry image will have the same size as the correctly focused image would.
Because their images have constant magnification and geometry, telecentric lenses are used for metrology applications, when a machine vision system must determine the precise size of objects independently from their position within the FOV and even when their distance is affected by some degree of unknown variations. These lenses are also commonly used in optical lithography for forming patterns in semiconductor chips.
Object-space telecentric lenses have an entrance pupil infinitely far behind the lens; that is, if one looks in the front of the lens, the image of the aperture is very far away.
Telecentric lenses tend to be larger, heavier, and more expensive than normal lenses of similar focal length and f-number. This is partly due to the extra components needed to achieve telecentricity, and partly because the object or image lens elements of an object or image-space telecentric lens must be at least as large as the largest object to be photographed or image to be formed. As of 2006, these lenses can range in cost from hundreds to thousands of US dollars or euros, depending on quality. Because of their intended applications, telecentric lenses often have higher resolution and transmit more light than normal photographic lenses.
In order to optimize the telecentric effect, these lenses are often used in conjunction with telecentric (or "collimated") illuminators, which produce a parallel light flow, often from LED sources