TECHSPEC® Telecentric Lenses can be found on pages 80-107
Section 5.1: The Advantages of Telecentricity
(center) and a Telecentric Lens (right). Note that in the image taken
with a Telecentric Lens, it is impossible to tell which object is in front
of the other. With the Fixed Focal Length Lens, it is quite obvious that
the object that appears smaller is positioned farther from the lens.
While Figure 5.2 is drastic in terms of a working distance shift, it
illustrates the importance of minimizing parallax error. Many automated
inspection tasks are imaging objects that move through the eld of
view of an imaging system, and the position of parts is rarely perfectly
repeatable. If the working distance is not identical for each object that
the lens is imaging, the measurement of each object will vary due to
the magni cation shift (see Section 2.2 on magni cation and how it is
de ned, page 13). A machine vision system that outputs di erent results
based on a magni cation calibration error (which is unavoidable
with a Fixed Focal Length Lens) is a non-reliable solution and cannot
be used when high precision is necessary. Telecentric Lenses eliminate
the concern about measurement errors that would otherwise occur
due to factors such as a vibrating conveyor or inexact part locations.
Figure 5.2 SETUP FIXED FOCAL LENGTH LENS TELECENTRIC LENS
imaging resource guide imaging lenses filters microscopy cameras illumination targets
Section 5: Telecentricity and
The ability to quickly perform repeatable, high accuracy measurements
is critical to maximize the performance of many machine vision systems.
For such systems, a telecentric lens allows the highest
possible accuracy to be obtained. This section discusses the
unique performance characteristics of Telecentric Lenses
and how telecentricity can impact system performance.
Zero Angular Field of View: Parallax Error Elimination
Conventional lenses have angular elds of view such that as the distance
between the lens and object increases, the magni cation decreases. This is
how the human vision behaves, and contributes to our depth perception.
This angular eld of view results in parallax, also known as perspective
error, which decreases accuracy, as the observed measurement of the vision
system will change if the object is moved (even when remaining within
the depth of eld) due to the magni cation change. Telecentric Lenses
eliminate the parallax error characteristic of standard lenses by having a
constant, non-angular eld of view; at any distance from the lens, a Telecentric
Lens will always have the same eld of view. See Figure 5.1 for the
di erence between a non-telecentric and a telecentric eld of view.
A Telecentric Lens’s constant eld of view has both bene ts and
constraints for gauging applications. The primary advantage of a Telecentric
Lens is that its magni cation does not change in respect to
depth. Figure 5.2 shows two di erent objects at di erent working distances,
both imaged by a Fixed Focal Length (non-telecentric) Lens
Figure 5.1: Field of view comparison of a conventional and Telecentric Lens. Note the conventional lens’s angular eld of view and the
Telecentric Lens’s zero angle eld of view.
Figure 5.2: The angular eld of view of the Fixed Focal Length Lens translates to parallax error in the image and causes the two cubes to
appear to be di erent sizes.