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introduction fundamentals lens specifications real world performance telecentricity lens mechanics lens selection guide
Section 6: Imaging Lens Selection Guide
Throughout the prior six 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.
Imaging lenses are a complicated and nuanced component in imaging
systems, and it is not always straightforward which decisions to
make when it comes time to choose a lens and what tradeoffs are made
as a direct result of those decisions. Lens specification sheets (or datasheets)
vary between manufacturers, which can make comparisons a
daunting task. Oftentimes, however, the problem is not as complicated
as that, as it can be challenging enough to determine even the type of
lens that is required for a particular 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 buckets: a Variable Magnification Lens or a Fixed Magnification
Lens. It describes these different types of lenses and what they are useful
for. 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. It discusses pixel mapping and contrast reproduction of
features on the pixel level of a camera.
It is important to read (or have prior understanding of) the previous
six sections of this text before jumping into this section, as it will
discuss previously learned about concepts without going into any background
detail. An understanding of the prior sections is particularly
important for 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. They are the most common types of lenses that exist in machine
vision; as a good rule of thumb, if a lens is referred to as a single
focal length (e.g., a 25mm lens), it 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 a fixed AFOV. 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 75mm focal length Fixed Focal Length Lens
focused at two different distances. While the spacing between each
element does not change as it 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
200mm 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
a “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.
Fixed focal length lenses should be used for the vast majority
of machine vision applications, as they are flexible and have
great performance. 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 the focal length of a lens
goes down, its AFOV increases. This occurs in a linear fashion; therefore,
a 25mm focal length lens will have an AFOV that is twice as large
as lens with a 50mm field of view FOV.
Because these lenses can be focused at different distances and have
different magnifications, they are classified as a Variable Magnification
Lens 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, it is 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 a zoom lens varies between an 8mm
and a 48mm focal length, it is said to be a 6X zoom lens (48mm/8mm =
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 a resolution as in a
comparably priced Fixed Focal Length Lens, as the complex mechanics
and optical elements are multitasking.
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