C, CS and M12 (S-mount) are the three most common lens mounts in industrial and security imaging. Their threads are close but flange distances differ, so mixing them can break focus. This guide clarifies the differences, adapters and precautions.
1. The three mounts compared
| Mount | Thread | Flange distance | Typical use |
|---|---|---|---|
| C-Mount | 1″-32 UN | 17.526mm | Industrial cameras, machine vision |
| CS-Mount | 1″-32 UN | 12.5mm | Security, some industrial cameras |
| M12 (S-mount) | M12×0.5 | ~12.5mm (structural) | Small/embedded cameras, board cameras |
2. Using a C lens on a CS camera
C and CS share the same thread, but the C flange distance is longer (17.526mm vs 12.5mm). Mounting a C lens on a CS camera requires a roughly 5mm C/CS spacer, otherwise it cannot focus at infinity. Conversely, a CS lens on a C camera usually cannot reach infinity focus.
3. M12 vs C / CS conversion
M12 uses an M12×0.5 thread, different from C/CS, so an M12-to-C/CS adapter ring is required. Check that the resulting flange distance and back focus fall within the focusing range.
4. Back-focus adjustment
If the image is soft or cannot focus after mounting, fine-tune the flange distance via the camera's back-focus ring. Fixed cameras rely on the machining accuracy of the lens and spacer.
5. FAQ
Are C and CS lenses interchangeable?
Not directly. A C lens on a CS camera needs a 5mm spacer; a CS lens on a C camera usually cannot focus at infinity.
Can an M12 lens be used on a C-mount camera?
Yes, via an M12-to-C adapter ring, provided the focusing range and image circle are adequate.
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