Why do some C-mount lenses only focus close-up and not reach infinity on my adapter?
Asked 8/6/2025
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2 answers
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I’m adapting several lenses labeled as C-mount on two different cameras. Some focus normally and reach infinity, but others only focus at very close distances, almost like macro-only lenses. Those lenses have infinity and distance markings on the barrel, so they seem like they should focus at normal distances. Since the same lenses have the problem on both adapters, I’m wondering if there is another standard that looks like C-mount but uses a different flange distance.
Originally by Austin West. Source · Licensed CC BY-SA 4.0
Austin West
10mo ago
2 Answers
4
Is there like a secondary standard for c-mount requiring a closer flange distance?
There is a CS mount which is identical to C mount on thread and diameter, but was made for smaller formats (1/2" and below) and that has a flange distance of 12.526mm vs. C mount's 17.526mm.
You can find 5mm spacer/adapters to convert CS mount cameras to take C mount lenses. But sinking a CS mount lens 5mm further into whatever mount you're adapting to is something you're not likely to find a solution for.
Originally by inkista. Source · Licensed CC BY-SA 4.0
inkista
10mo ago
0
Generated from our catalog & community — verify before relying on it.
Yes — the likely issue is CS mount, which uses the same thread as C-mount but a shorter flange focal distance. C-mount is 17.526mm, while CS mount is 12.526mm, a 5mm difference.
A CS-mount lens can screw into a C-mount adapter because the threads match, but it will sit too far from the sensor. That prevents it from reaching infinity and makes it behave like it only focuses very close, even if the lens has normal distance markings.
This also explains why the same lenses fail on multiple adapters while other C-mount lenses work fine.
Cameras with CS mounts can usually take C-mount lenses by adding a 5mm spacer. Going the other way is the problem: a CS lens generally needs to sit 5mm closer to the sensor, which most adapters won’t allow. So if a “C-mount” lens won’t reach infinity, it may actually be a CS-mount lens or otherwise designed for a shorter registration distance.
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AI10mo ago
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