with a 12mm lens (Figure 1). Here, the maximum theoretically possible contrast that can
be achieved is almost 70% at 150 lp/mm and
no lens design can perform higher (http://bit.
ly/VSD-M TF). Thus, choosing a high-performance megapixel camera matched to a lens
with poor resolving power or an inexpensive
camera with a high-performance lens will
both result in low contrast images.
Fixed or varifocal?
Fixed focal lenses used in machine vision systems are prevalent since they have the benefits
of using fewer optical components, low optical distortion and are relatively inexpensive.
In some applications, however, the FOV may
need to be modified, especially if the system
design may change over time or if a systems
integrator needs to determine a suitable focal
length required for an application. In such
cases, a varifocal length lens can be chosen.
Unlike zoom lenses that maintain focus as
the focal length is changed, varifocal lenses
require refocusing while allowing different
FOVs to be imaged.
As pointed out by Pete Kepf of CBC/Com-
putar (Cary, NC, USA; https://computar.com)
in his white paper “Seven Important Factors
When Selecting a Machine Vision Lens,”
( http://bit.ly/VSD-7F), varifocal lenses have
an adjustment that changes the magnifica-
tion and FOV for a given working distance
and typically have adjustments for the lens
aperture, focal length and focus. Unlike fixed
focal length lenses, varifocal lenses are spec-
ified across different focal lengths, for exam-
ple, 3.5-8mm or 6.5-52 mm, meaning that the
focal length of the lens can be can be adjusted
within these ranges.
Such lenses are more expensive than fixed
focal length lenses since they require a middle
or rear assembly of elements that slide back and
forth and a separate typically front cell focus
adjustment mechanism. However, as camera
vendor Lumenera (Ottawa, ON, Canada; www.
lumenera) points out in “Selecting a Lens for a
Vision System,” if the camera used in a system
has been previously selected, it may be wise to
consider purchasing an inexpensive (low qual-
ity) varifocal lens to determine a suitable focal
length ( http://bit.ly/VSD-Sel).
Unlike varifocal lenses that require refo-
cusing, zoom lenses or parfocal lenses main-
tain focus as the focal length is changed.
While many companies use the term zoom
lenses to describe their products, many of
these are actually varifocal lenses that require
For example, the MLH-3XMP lens from
Moritex (San Jose, CA, USA; www.moritex.
com) that features a 0.3 to 1x magnification
and a working distance of 90mm, is billed as
a zoom lens on the company’s website. How-
ever, it is actually a varifocal lens since it fea-
tures a variable focus setting. Properly named
as a zoom lens, the Zoom 7000, a close-focus-
ing manual zoom
lens designed to
image objects over
1in in diameter
from Navitar (Roch-
ester, NY, USA; https://navi-
tar.com) is parfocal over its
zoom range, such that cap-
tured images remain in focus
over the zoom range without
While the use of zoom lenses is less
common in machine vision applications,
they are used in applications such as microscopy to provide either manual or motor-con-trolled driven image magnification. By using
such zoom lenses, systems integrators free
operators of removing and replacing lenses to
inspect the part at the magnification required
or mounting numerous types of lenses in a
turret that allow magnification to be changed
without handling the lens. Using a zoom lens,
the inspection system can be automated such
that the system can be programmed to view a
scene at low magnification and zoom in on specific details without changing lenses or rotating a lens turret (see “Understanding Optical
Inspection,” Quality Magazine, August 2009;
Such zoom lenses can be either manually
or automatically adjusted via computer control. The design of Fetura lenses, for example, Excelitas Technologies (Waltham, MA,
USA; http://www.qioptiq.com) uses two DC
motors to drive both a variator lens and a compensator lens along a linear axis. While the
variator lens varies the magnification factor of
the image, the compensator lens compensates
for the displacement of the image to bring the
object being viewed into focus. (see “Zoom
lenses target machine vision,” Vision Systems
Design, June 2008; http://bit.ly/VSD-ZOOM).
Figure 3: Since the size of pericentric
lenses can become large, the size and
weight of such lenses can be reduced
using catadioptric lenses such as the
PCCD series from Opto Engineering
to image top and sides of a product simultaneously.
Figure 2: Two screws
imaged with (side) a conventional lens highlighting
the perspective error that
would result in incorrect
height measurement and
(above) the same image
using a bi-telecentric lens.