Imaging Lens Selection Guide
This is Section 7.4 of the Imaging Resource Guide
Imaging lenses are a complicated and nuanced component in imaging systems. 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 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 prime lens the best choice? What about a zoom lens? Or a telecentric? This section will explain the differences between the different types of lenses, and how best to choose what is required for any given application.
Section 7.4.1: Variable Magnification Lenses
Fixed Focal Length Lenses
As explained in Understanding Focal Length and Field of View, fixed focal length lenses have a fixed angular field of view. These lenses can still focus at different working distances, which is most often achieved by moving all of the individual lens elements together such that the relative spacing between them does not change. Fixed focal length lenses should be used for the vast majority of machine vision applications, as they are flexible and have great performance. Fixed focal length lenses are also known as prime lenses (a term that comes from photography).
Figure 1 shows a 75mm focal length fixed focal length lens focused at two different distances. While the spacing between each element did not change as it was focused, 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 200mm away from it (a 200mm working distance).
Figure 1: A 75mm Double Gauss-type fixed focal length lens focused at two different working distances. Note that the spacing between each element did not change as working distance shifts.
It is important to remember that true fixed focal length lenses will always behave as in Figure 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.
Where fixed focal length lenses are designed to have a fixed angular field of view, zoom lenses are designed to change their focal length, and hence their fields of view. Zoom lenses are ideal for applications that require the ultimate amount of flexibility during use and do not require high resolution; unless a field of view 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 2 shows the same zoom lens set to two different focal lengths. Note that both relative element spacing and distance to the image plane changes, despite the fact that the working distance 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. Zoom lenses not only attempt to get the best performance at a single focal length, but they are required to function over a broad range of focal lengths, which lowers the overall performance.
Figure 2: A zoom lens at multiple optical magnifications.
Other interesting optical properties may arise as a direct result of the complex movements in a zoom lens, such as depending on how the lens was designed, the f/# can change as the focal length changes. This type of design is typically avoided for photography or videography lenses, but for machine vision lenses this is often not the case. It is also important to recall from f/# (Lens Iris/Aperture Setting) that the working f/# will still change as the magnification changes, resulting in different exposures.
By definition, as zoom lenses change their field of view, they remain in focus. If a lens is defocused as its focal length is changed, it is more accurately referred to as a varifocal lens, not a zoom lens.
Macro lenses can be thought of as a subset of fixed focal length lenses where the magnification is around 1X (the sensor is the same size as the object) or a bit greater, and the working distance is relatively small. Due to their high magnifications, macro lenses tend to run at an f/# generally around a factor of two larger than what is stated on their barrels (see f/# (Lens Iris/Aperture Setting) for more information on working f/#). Often, typical fixed focal length lenses can be turned into macro lenses with the use of spacers (see Lens Spacers, Shims, and Focal Length Extenders), or by reversing them such that they image backwards. See Figure 3 for a lens in macro configuration and the resulting image.
Figure 3: A 50mm fixed focal length lens paired with approximately 70mm extension length, yielding a 1.25X magnification at a 33mm working distance.
Section 7.4.2: Fixed Magnification Lenses
Telecentric lenses should be used any time a high accuracy measurement in a system needs to take place. They are highly specialized, fixed magnification lenses that come with many powerful optical capabilities. The working principles and advantages of telecentric lenses are discussed in detail in Telecentricity and Perspective Errors.
The selection of a telecentric lens is often thought of as more challenging than that of a fixed focal length lens, though this is almost always not the case. For details on telecentric lens and fixed magnification lens selection see How to Choose a Fixed Magnification Lens.
Microscope objectives are used to image very small objects, generally with magnifications much greater than 1X. They are fixed magnification optics that only function properly at a single working distance, which is generally quite small relative to other imaging lenses. Microscope objectives should be used when a high magnification image is required and no strict minimum working distance constraints.
Objectives have various data written on their barrels to describe certain optical properties of the objective, which gives the user clues on best uses. Figure 4 shows an image of a 20X optical magnification infinity corrected (infinite conjugate) microscope objective designed for a 0mm cover glass thickness and a 200mm focal length tube lens, which is required to focus it. The objective has a numerical aperture of 0.42 (for more information on numerical aperture, see f/# (Lens Iris/Aperture Setting)). For more detailed information on microscope objectives, see Using Infinity Corrected Objectives.