When standard lens solution are not providing the optimal field of view at the desired working distance, lens adapters can be used. On the bottom there is an explanation how lens adapters work.
C/CS mount to M12 adapters
The C or CS mount to M12 lens adapters are used to mount M12 lenses on a C-mount or CS-mount camera. M12 lenses are cheaper, compacter and have less weight. In some applications there parameters are critical. Without this adapter it's not possible to mount a M12 lens on a C or CS-mount industrial camera.
M12 or C-mount extension rings
Extensionrings are used to shorten the minimum working distance of a C-mount or M12 lens. Most industrial lenses have a working distance of 100mm or higher. By adding extension rings, this distance can be reduced. When reducing the working distance, the depth of field is also reduced. Adding extension rings willl reduce the optical performance of the lens and the light sensitivity. Therefore M12 or C-mount extension rings are only used in specific industrial applications.
Focal length extender
With a focal length extender you can extend the focal length of the lens by a factor 2 or 4. By using this focal length extender in combination with a 35 or 50 or 75mm c-mount lens you can make focal lenghts of 70mm, 100mm, 150mm, 200mm and 300mm.