C-mount is a threaded lens mount standard used on box-style and specialty cameras, with a one-inch thread and a flange distance of 17.526 millimeters. It lets a camera body take interchangeable lenses, so one camera can serve a wide lobby view or a long entrance lane. CS-mount bodies use the same thread with a shorter flange distance.
Interchangeable mounts are how a camera body is matched to a job that needs a specific view. The mount standard decides which lenses physically fit, and the flange distance decides whether they focus.
C-mount and CS-mount
Both use a one-inch thread, but a CS-mount places the lens closer to the sensor. A C-mount lens on a CS-mount camera needs a five-millimeter adapter ring, and a CS-mount lens on a C-mount camera will not reach focus. Mixing the two without checking is a common mistake.
Choosing the lens
Match the lens to the sensor size, or the image vignettes at the corners. Use an infrared-corrected lens if the camera switches to infrared at night, and choose a lens rated for the camera’s resolution. A poor lens wastes a good sensor.
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