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Optics / Lenses / Filters Questions

Question Asked:

THE OPTIMAL WORKING DISTANCE I want to inspect an electronic board with the size of 100 mm x 80 mm, therefore I would like to have a Field of View of 128 mm x 102 mm. The camera that i chose has a sensor with the size of 6,784mm x 5,427 mm (This size is calculated from the resolution and the pixel size of the camera). I want to find the optimal working distance for this camera, which can be calculated with the following formula: Focal length = Working distance * (sensor size/Field of View). If a fix the working distance at 250mm, I shall get the following calculation: Focal length = 250 mm * (6,784mm/128mm) = 13,25 mm. If the working distance changes, the focal length changes too. This make me wonder if there is an optimal choice of the working distance and the focal length so that I can obtain a better image quality with a non-expensive lens ? Some lenses are characterized by the Min Operating Distance (MOD), what will happen if I take a picture from a distance smaller than the MOD ? Thanks for any anwser.

2 Answers

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Perry West - President
Automated Vision Systems, Inc.
perry [at] autovis [dot] com
(408) 267-1746

As a rule, if you use lens extension tubes to focus closer than the MOD, there is a risk of degradation of lens resolution -- you may experience a loss of sharpness in the image. Some lenses can be extended up to 50% of their focal length (e.g., using a 25mm extension on a 50mm lens) without serious degradation. Other lenses will be seriously degraded when used closer than the MOD. As a technical point, the focal length of a lens does not change as you change the focus, only the image distance changes. As for an optimal working distance, I'd recommend a working distance between the field-of-view diagonal and two times that diagonal. If you need to be closer due to mechanical limitations, then the first choice would be to look for a macro lens that meets your requirements. A second choice (less desirable), is to fold the optical path to fit the available space using mirrors.

Brian Durand - President
I4 Solutions, LLC
bdurand [at] i4solutions [dot] us
(612) 888-6000

Hello, So long as the the working distance is greater than the MOD, you'll be fine. Choose the working distance that is convenient for your installation. That said, you'll notice that lenses having a shorter focal length will result in some "fish-eye" distortion. Basically the size/shape of an object will change as it moves from center of image toward corner of image. If you're measuring components on the electronic boards, then consider a telecentric lens. You'll find that the lens aperture has an effect on image quality. Most machine vision lenses perform best when set around f4, assuming there is sufficient light. Set the camera exposure time accordingly. Be careful to choose a lens designed for the size of the pixels in your camera. Megapixel ratings don't really mean much, as there is no standard. You may find this page helpful. It answers common questions and links to several lens calculators: https://machinevisionstore.com/tech/lenses Good luck with your project.

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