Telecentric vs. Conventional Lenses

A conventional (entocentric) lens changes magnification with object distance: the closer the part, the bigger it images. That is perspective error. In dimensional gauging, a few millimetres of height variation or placement offset is enough to introduce percent-level errors.

How telecentric optics remove the error

In an object-space telecentric lens the chief rays are parallel to the optical axis on the object side, so magnification stays constant across the whole depth of field and moving the part does not change its imaged size. Bi-telecentric designs are telecentric on the image side as well, further reducing sensitivity to camera mounting.

When a conventional lens is still fine

Alignment, barcode reading and presence/absence checks do not depend on absolute dimensions, so a cost-effective FA fixed focal lens is the right tool.