Precision lenses do an excellent job conveying contrast and color from object to image, but they can do a poor job of transferring accurate dimensions, especially if they have small focal lengths. Traditional lenses work in the same way as the human eye: nearer objects appear larger than farther objects. This change in magnification with respect to distance is partially what allows for depth perception, but it will reduce the accuracy of a machine vision system that is being used for precise measurements. If a vision system requires the most accurate possible measurement, it is imperative to ensure that the relationship between an object’s dimensions and the corresponding image’s dimensions is well-defined and unchanging. For these systems telecentric lenses are required.
In practice, no telecentric lens is perfect. That is, no telecentric lens images only light entering parallel to the optical axis.