resource guide telecentric liquid lens/specialty objectives cameras illumination targets
fixed focal length filters/accessories microscopy /
www.edmundoptics.eu/imaging 37
introduction fundamentals lens specifications real world performance telecentricity lens mechanics lens selection guide
Section 6: Imaging Lens Selection Guide
Throughout the prior five sections of this text, an understanding of imaging
lenses has been slowly built up, piece by piece. From understanding
the basics of angular field of view (AFOV) and resolution (Section
2: Understanding Lens Specifications on pages 10-19) to learning about
modulation transfer function (MTF) and what variables impact lens performance
(Section 4: Real World Performance on pages 20-28), the story of
how a lens fundamentally behaves has been laid out. This section aims
to bring everything from the prior sections together for the confident
selection of an imaging lens for any given machine vision problem.
Because imaging lenses are complex, nuanced components, making
decisions about about the selection of a lens based on performance
tradeoffs is not always straightforward. Lens specification
sheets (or datasheets) vary between manufacturers, which can make
comparisons a daunting task. Even before addressing lens specifications,
one needs to consider what type of lens is best for the application.
Is a fixed focal length lens the best choice? What about a zoom
lens? Or a telecentric?
This lens selection guide is broken into three distinct parts. Section
6.1 answers the question: what kind of lens do I need for my application?
For the purposes of this text, lenses will be divided into one of
two useful descriptions: variable magnification lenses and fixed magnification
lenses. Section 6.2 then explains how to make an informed
lens selection if a camera has already been chosen, which is often the
case for many applications. It focuses primarily on field of view (FOV)
matching given constraints such as working distance (WD). The final
piece, Section 6.3, explains how to choose a lens alongside the camera,
which is important if the application is to be properly optimized
in terms of cost and performance. This section discusses camera-level
pixel mapping and the subsequent necessary contrast requirements.
Please read or become acquainted with the previous five sections
of this text before beginning this next section as the proceeding topics
will be discussed with little background explanation. An understanding
of prior sections is particularly crucial to understand the contents
of Section 6.3.
Section 6.1: Types of Machine Vision Lenses
Types of Variable Magnification Lenses
Fixed Focal Length Lenses
Fixed focal length lenses are known by many different names: prime
lenses (common in photography or cinematography), FA lenses (where
FA typically stands for factory automation), or simply as machine vision
lenses. These lenses are the most common types for machine vision.
Note: If a single focal length is used to refer to a lens (e.g. 25 mm lens),
this lens is typically a fixed focal length lens. As explained in Section 1.3:
Understanding Focal Length and Field of View on pages 8-9, fixed focal
length lenses have fixed AFOVs. These lenses can still focus at different
WDs, which is most often achieved by moving all of the individual lens
elements together such that the relative spacing between them does
not change.
Figure 6.1 shows a 75 mm fixed focal length lens focused at
two different distances. While the spacing between each element
does not change as the lens focuses, the distance between the image
plane and the last lens element varies a great deal. The top lens
is focused at optical infinity, and the bottom lens is focused at a
200 mm WD.
It is important to remember that true fixed focal length lenses will always
behave as in Figure 6.1, though some lenses exist that have “floating
element focus,” where the relative element spacing does change
through focus. This spacing change does impart a change in the focal
length of the system, though it is usually not enough to classify them
differently.
Because they are flexible and have great performance, fixed
focal length lenses should be used for the vast majority of machine
vision applications General parts inspection, barcode reading,
biometric and document scanning, license plate reading, and other
types of optical character recognition (OCR) or optical character verification
(OCV) are all best suited for fixed focal length lenses most of the
time.
The AFOV that a fixed focal length lens has is a product of its focal
length matched with its sensor size. As focal length decreases, AFOV
increases linearly. Therefore, a 25 mm focal length lens will have an
AFOV twice as large as a lens with a 50 mm focal length.
Because these lenses can be focused at different distances and have
different magnifications, they are classified as a variable magnification
lenses for the purpose of this document.
Zoom Lenses
Where fixed focal length lenses are designed to have a fixed AFOV,
zoom lenses are designed to change their focal length, and hence their
AFOV. Zoom Lenses are ideal for applications that require the
ultimate amount of flexibility during use and do not require
high resolution; unless a FOV actively needs to change while
imaging, zoom lenses are likely not the best choice. When this is
the case, stepper motors are required to change the focal length quickly
and accurately.
Zoom Lenses are specified as having particular zoom ratios, which
can be found by dividing the longest focal length option by the smallest
for any given lens. For example, if the focal length of a zoom lens
varies between 8 mm and 48 mm, then it is said to be a 6X zoom lens
(48 mm/8 mm = 6X). This can also be expressed as a ratio: for the
aforementioned lens it would be a 6:1 zoom ratio.
Figure 6.2 shows the same zoom lens set to two different focal lengths.
Note that both relative element spacing and distance to the image plane
change, despite the fact that the WD has not changed. These complicated
mechanics add to the cost of the lens system, as precise movements
are required to simultaneously change the lens’s focal length and
keep it in focus. Also, zoom lenses cannot have as high of a resolution
as a comparably priced fixed focal length lens, as the complex mechanics
and optical elements have been designed for many use cases rather
than optimized for a single case.
Figure 6.1: A 75mm Double Gauss-type fixed focal length lens focused
at two different WDs. Note that the spacing between each element
did not change as WD shifts. (Continued on page 38)
/imaging