Telecentric lens

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 front element in an object-space telecentric lens is often much larger than the camera mount.

In contrast to entocentric lenses where lenses are made larger to increase the aperture for increased collection of light or shallower depth of field, a larger diameter (but otherwise similar) object-space telecentric lens is not faster than a smaller lens.

A truly telecentric lens has no focus ring to adjust the position of the focal plane.

This allows the lens to focus light from an object or sample to different distances without changing the size of the image.

Since the chief rays (light rays that pass through the center of the aperture stop) after an image-space telecentric lens are always parallel to the optical axis, these lenses are often used in applications that are sensitive to the angle of incidence of light.

Interference-based color-selective beam splitters or filters but also Fabry–Pérot interferometers are two examples where image-space telecentricity is used.

[1] Since every pixel is illuminated at the same angle by an image-space telecentric lens, they are also used for radiometric and color measurement applications, where one would need the irradiance to be the same regardless of the field position.

Large and heavy bi-telecentric lenses with many optical elements are commonly used in optical lithography (that copies a template of an electrical circuit to print or fabricate onto semiconductor wafers for mass semiconductor device production) because small image distortion and placement errors can be critical for manufactured device functionality.

Bi-telecentric lens with 208 mm diameter front element and a C-mount camera interface
3D render simulating a photo taken with a fixed focal length and a telecentric lens
Object-space telecentric imaging where the aperture is in the back focal plane of the objective. The entrance pupil is located at infinity, and chief rays before the objective are parallel to the optical axis.
Image-space telecentric imaging where the aperture is in the front focal plane of the objective. The exit pupil is located at infinity, and chief rays after the objective are parallel to the optical axis.
Bi-telecentric imaging where the aperture is in the common focal plane of two confocal lenses.