How Do I Focus C-Mount Lenses?

C-mount lenses require a different focusing approach due to their standardized flange focal distance.
They typically have a fixed 17.526mm flange focal distance.

You focus by rotating the lens's focus ring - not by threading the whole lens like M12.

C-Mount Focus Procedure

1. Mount lens to camera:

Thread C-mount lens fully into mount until flange contacts. This sets the precise 17.526mm spacing.

2. Set aperture:

Open to maximum aperture for brightest view during focusing, then stop down to working f-number after focusing.

3. Rotate focus ring:

Turn the focus ring (not the entire lens) while viewing live image. Most C-mount lenses have ~270° of rotation.

4. Verify across field:

Check center and corners at working aperture. Industrial C-mount lenses should maintain <5% MTF variation.

5. Lock focus ring:

Tighten set screws on focus ring or apply removable thread locker to prevent drift during operation.

The Imaging Source offers a broad selection of C-mount lenses optimized for 1/1.8" to 1.2" sensors, with focus rings designed for smooth adjustment and reliable locking.

For more information refer to our C-Pro and C-Ultra lens series.