The camera C-mount can be adjusted to match the back-focus of the lens to the focal plane of the sensor. This adjustment is made at the factory and should not require adjustment in the field. However, if the back focal plane of your lens does not conform to the C-mount back-focus specification you may need to make this adjustment in the field.
To make this adjustment you will need to use a lens with an infinite focus setting (or at least a known focus setting). Connect the lens and make sure that it is fully seated on the C-mount.
Loosen the hexagonal c-mount locking ring.
With the lens set to infinity focus (or a known focus distance), set the camera to view an object located at 'infinity' (or the known distance).
Rotate the silver c-mount ring and lens until the object is in sharp focus being careful that the lens remains seated in the C-mount.
Tighten the hexagonal c-mount locking ring.