Image-space telecentric lenses
An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image. Image-space telecentric lenses have an exit pupil infinitely far in front of the lens; that is, if one looks in the back of the lens, the image of the aperture is very far away.
At the film or image sensor, all of the chief rays from these lenses hit "straight on", or at zero angle of incidence. This property minimizes any angle-of-incidence dependence of the sensor, or of any beam-splitter prism assembly behind the lens, such as a color separation prism in a three-CCD camera. Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, to avoid the vignetting and color crosstalk that occur in color filter array-based digital image sensors with oblique incident rays. The Four Thirds System uses this approach.
Since the ray cones approaching the detector surface have the same angle of incidence and angular subtense everywhere in the image plane, the image is evenly illuminated. This feature is commonly used in photography and is very useful for radiometric and color measurement applications, where one would need the irradiance to be the same regardless of the field position.
Lenses that are double-telecentric have magnification that is more precisely constant than those that are only object-side telecentric, because the principal ray intercept position on the detector does not change. This property allows precise measurement of objects regardless of position.