What’s the difference between working distance, minimum focus distance, and infinity focus on a machine-vision lens?
Asked 6/12/2024
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I’m new to lenses and trying to understand how focal length relates to focusing distance on machine-vision lenses. Some datasheets list values such as a working distance of 200 mm and a minimum focus distance of 50 mm. I’m confused about how that relates to infinity focus.
My questions are:
- Why might a lens not be able to focus at infinity?
- For a lens with a minimum focus distance of 50 mm and a stated working distance of 200 mm, how should I interpret those distances?
- Why do lens manufacturers specify lens resolution at all, instead of it being effectively perfect or infinite?
Originally by Aravind D. Chakravarti. Source · Licensed CC BY-SA 4.0
Aravind D. Chakravarti
2y ago
2 Answers
2
- No its because the lens cannot move close enough to the sensor(*). Its the same problem as SLR cameras using a macro extender rings on the lens. The camera can focus closer, but it's "far" focus is no longer at infinity.
- No because to focus at 200mm the lens is closer to the sensor than when focusing at 50mm.
- If the lenses where actually thin lenses and used with small angles (in other words for a very small area at the center) and monochrome light then yes, but then there is this pesky real world where the refractive index depends on wavelength and users want wide fields so you have to combine multiple elements and even then you cannot get a perfect focus which reduces the attainable resolution.
(*) See Wikipedia. To focus to a close object (smaller S1), the lens has to be farther from the object plane (wider S2).
Originally by xenoid. Source · Licensed CC BY-SA 4.0
xenoid
2y ago
0
Generated from our catalog & community — verify before relying on it.
Infinity focus happens when the lens is positioned for very distant subjects, so the image forms at the sensor plane. To focus closer, the lens must move farther from the sensor. If a lens is set up only for close working distances, it may no longer be able to move close enough to the sensor to reach infinity focus.
So for your example, a minimum focus distance of 50 mm means the closest subject distance it can focus on is 50 mm. A stated working distance of 200 mm usually means that lens is intended or optimized to be used around 200 mm, not necessarily that it only focuses from 50 mm to 200 mm.
Also, your diagram idea is only an idealized thin-lens approximation. Real lenses use multiple elements, wider fields of view, and work with non-monochromatic light. Because of aberrations and wavelength-dependent refraction, real lenses cannot form a perfectly sharp point everywhere in the image.
That is why manufacturers specify resolution: it describes the real detail the lens can deliver, which is always limited by optical design and conditions rather than being infinite.
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