Application Note 2 Lens Mountings Introduction There are two kinds of lenses commonly used with high-speed video equipment. Wouldn't you know it – they have similar names. One is called C-mount and the other one is called CS-mount. To make matters worse (as if the terminology wasn't confusing enough), the two types of lenses are very similar -- almost identical. Almost, but not quite. Which is Which? C-mount lenses are somewhat more common. They mount with a 1-32 thread (one inch in diameter and 32 threads per inch). CS-mount lenses also use a 1-32 thread. So you can physically install a C-mount lens on a CS-mount camera, and vice-versa. But will it work? Only sometimes. Flying on Focal Planes The difference between the two standards has to do with focusing. Every lens is designed to be a certain distance from the imaging element -- whether that imaging element is a video sensor or film. This distance specifies the focal plane. C-mount lenses are designed for a focal plane distance that is 5 mm greater than the same dimension for CS-mount lenses. In other words, C-mount lenses are designed to be 5 mm x.xxx inches) farther from the sensor than CS-mount lenses. What does it Mean? If you have a C-mount camera You must use C-mount lenses. CS-mount lenses will NOT work with your camera. An adapter will not help. Find a C-mount lens that gives you the performance you need (for help, contact us). If you have a CS-mount camera: CS-mount lenses work fine with your camera. No adapter is needed. Just install the lens directly onto the camera/sensor unit. C-mount lenses will work with your system, but ONLY with an adapter. Install the C-mount lens to CS-mount camera adapter between the lens and the camera (aka the sensor, the sensor housing, the sensor unit, the imager, etc.). What is an Adapter? An adapter can make a C-mount lens work with a CS-mount camera. It is a spacer with both male and female 1-32 threads that moves a lens 5 mm farther from the camera. It is commonly made of either plastic or metal. Sensor Size Video sensors are the semiconductors (usually CCDs – charge coupled devices) that detect light patterns and turn them into electrical signals that can eventually be reassembled into images that can be displayed on monitors and printed. These sensor chips, like all semiconductors, have been shrinking in size as well as improving in performance. C-mount lenses are commonly used with video sensors that are one inch and smaller in diagonal size. CS-mount lenses are generally available only for sensors of 1/3 inch and smaller in size. A lens creates an image of a certain size on its focal plane. That size is part of the lens specification; it is expressed in terms of the sensor diagonal dimension. It is all right to use a lens designed for a larger sensor; i.e., to use a 1 inch lens with a 2/3 or 1/2 inch sensor. But using a lens designed for a smaller sensor (e.g., using a 1/2 inch lens with a camera with a 1 inch sensor) will bring unsatisfactory results. The image will be projected by the lens onto only part of the sensor, resulting in a reduction of resolution and image size. Contact us for further information.

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