Telecentric Lens
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. Telecentric lenses are often used for precision optical two-dimensional measurements or reproduction 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 makes the chief rays (light rays that pass through the center of the aperture) on the other side of the lens parallel to the optical axis for any 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 ...
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Entrance Pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil. If there is no lens in front of the aperture (as in a pinhole camera), the entrance pupil's location and size are identical to those of the aperture. Optical elements in front of the aperture will produce a magnified or diminished image that is displaced from the location of the physical aperture. The entrance pupil is usually a virtual image: it lies behind the first optical surfa