Telecentric lenses accept ray cones where the "principal ray" is
parallel to its mechanical axis because the entrance pupil is
placed at infinity. For this reason an orthonormal view of the
object (i.e. where no image of the object sides is present) is
frequently possible.
Standard lenses generate images of different size if a same
object changes its distance from the lens. On the other hand
objects of different size can be viewed as if they had the same
dimension, if they subtend the same viewing angle. In a telecentric
system rays get into the optics only with an almost
parallel-to-the-axis path. This effect is due to the specific path
of the rays: in the case of common optics the geometric information
"parallel" to the main optical axis shows a component on the
detector plane direction, while in a telecentric lens this
perpendicular component is not present at all.
You can think as if common lenses would build a correspondence
between the 3-dimensional object space and the 2-dimensional
detector (image) space: in the case of a telecentric lens the third
dimension in object space is not displayed. Some interesting
information can be found reading this tutorial online:
http://www.opto-engineering.com/telecentric.php
