Telecentric lenses represent a special class of optics designed to only collect collimated light ray bundles (i.e. parallel to the optical axis, see picture), thus eliminating perspective errors.
Since only rays parallel to the optical axis are accepted, the magnification of a telecentric lens is independent of the object location. This unique feature makes telecentric lenses perfectly suited for measurement applications, where perspective errors and changes in magnification can lead to inconsistent measurements. Because of its design, the front element of a telecentric lens must be at least as large as the desired FOV, making these lenses inadequate to image very large objects.
The drawings show the difference between common optics (entocentric) and telecentric lenses. Fixed focal length lenses are entocentric lenses, meaning that they collect rays diverging from the optical axis. This allows them to cover large FoVs but since magnification is different at different working distances, these lenses are not suited to determine the true dimensions of an object.