Q&A: Lens Basics Ask Brent Senior Application Engineer 1-888-3-SENSORS Description: The lens you choose can be an important piece of the vision inspection puzzle. Here are some concepts that will start you on your way to understanding how optics affect your inspection. Q: What type of lenses do you offer? A: We offer three types of c-mount lenses: standard, high performance, and megapixel lenses. Banner offers these lenses in the following focal lengths: 3.5, 4, 6.5, 8, 12, 16, 25, 35, 50 and 75 mm. We also have a 10-40mm zoom lens. Q: What is a c-mount lens? A: C-mount is a lens with a threaded mount. The threads have a major diameter of 1" and a pitch of 32 threads per inch. The flange focal distance is 0.69". C-mount lenses are used extensively with closed-circuit televisions, thus the lenses are readily available. Most machine vision sensors use a c-mount lens, although some use a slightly different standard, the cs-mount. The cs-mount lens is dimensionally the same as the c-mount except for the flange-to-focal plane distance (~0.49" for the cs-mount). Q: What’s the difference between standard, high-performance, and megapixel lenses? A: In a simple presence/absence application, a standard lens in the right focal length is a good choice. Standard lenses are inexpensive and are adequate for many applications. The high-performance and megapixel lenses are of better construction (metal housings rather than the plastic of the standard lenses) and most have the ability to adjust the aperture and to lock both the aperture and focus settings in place. By being able to adjust the aperture, you have more options as to where you place the part. By being able to lock the settings in place, you reduce the chance that vibration will eventually cause out-of-focus problems. When working with gauging and pattern matching applications, choose a megapixel lens because it offers less image distortion. Optically and mechanically, there is little difference between the high-performance and megapixel lenses, so we usually recommend the megapixel lenses. Q: Can I use a standard lens on a high-resolution sensor? A: Some standard lenses will work with a high-resolution sensor. In general, we recommend the use of a megapixel lens because the high-resolution sensor may highlight optical defects in a standard lens. Q: What is “field of view?” A: Field of view describes the area that is captured by the lens. Q: What is “working distance?” A: In a given lens setup, working distance describes the distance between the camera flange and the part under inspection. Q: Why is focal length important? A: Focal length relates the working distance to the size of the field of view (FOV). All lenses are specified using focal length. If you know your FOV and your working distance, you'll be able to determine what focal length lens you need. Q: Does the imager size determine what lens I need? A: Yes, the lens you choose must be capable of drawing an “image circle” as big or bigger than the imager chip of the vision sensor you are using. Q: How does the size of my inspection area affect the lens I should choose? A: The size of the inspection area (field of view) and the distance between the part and the camera (working distance) determine the “focal length” of lens. Focal length as you recall is used to specify a lens. Download these charts for help finding a lens by FOV and imager size. Q: How large an area can I inspect at close range? A: In general, the widest angle lenses we sell have about a 1:1 relationship between the inspection area and the distance from the part to camera. This results in a 1 foot inspection area for 1 foot distance between the part and the sensor.

Q: How does the aperture affect my application? A: By increasing or decreasing the size of the aperture, you let more or less light into the camera. This will affect the depth of field, that is, how much area behind and in front of the target remains in focus. The larger the aperture, the shallower the depth of field. The smaller the aperture, the deeper the depth of field. In some applications, you may want a very shallow depth of field in order to improve the contrast. However, aperture can affect other parts of your inspection. A small aperture may require a greater exposure time which in turn could slow down a high speed inspection. Q: How do I find the lens I need? A: You can search Banner’s Lens Product pages to see our selection of lenses. We also offer a tutorial on choosing a lens. Or you may always call in to a Banner representative for advice with your application. Q: Can I use a non-Banner lens on my Banner vision sensor? A: Yes as long as it is a c-mount lens, and can draw a big enough image circle to cover the size of the imager chip in your Banner vision sensor. Q: How can optics affect contrast in the application? A: The lens collects and focuses light on the surface of the imager chip, and as a result, can influence the quality of the image by how it focuses light. The better the resolution capability of a lens, the better it will be able to create crisp boundaries between light and dark objects. Q: Will the lens affect the performance of any of the vision tools? A: Yes, but only to the extent that the quality of the image is determined by your choice of lens. There’s a saying in the vision community “if you can’t see it in the image, then the vision sensor will not be able to see it either.” Some vision tools may encounter problems if their ROI touches the edge of the image. This means that you should make your field of view generous enough to accommodate the part, the normal movement of the part, and additional space for drawing with the software tools. Q: Will my choice of lens affect sensor response time? A: No. Response time is related to the number of physical photosites on an imager chip. The magnifying power of the lens will do nothing to change how many pixels are being processed. Q: Does the quality of the optics matter as long as I choose the right focal length? A: Yes, the quality of the optics does matter. A distorted image can cause problems for some image processing tools, and will certainly have a negative effect on accuracy. All Banner’s standard lenses are of good quality. Q: Are your sensor-mounted lights compatible with the lenses? A: In general, all of our standard lenses from 4 mm to 16 mm fit the ringlights, and all high-performance and megapixel lenses fit all the ringlights. Q: I need to mount my sensor very far from my application. What can I do to extend my working distance? A: Depending on how greatly you need to extend the working distance, you may have to purchase a specialty lens from a third-party supplier. Banner has used and recommends lens from Edmund Optics for very long distance applications. Q: I need a very small inspection area, what can I use? A: Banner has used and recommends video microscopes from Edmund Optics. Sub-millimeter fields of view are possible. Related Links: For more detailed information, please review this lens tutorial. Download this training note on selecting a lens.