Understanding C-Mount and CS-Mount Standard Mechanics, Optics, and Cameras
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7u The C-mount standard is a widely-used method of connecting connecting small cameras to lenses, such as mounting a lens on an industrial camera, or for connecting trinocular phototubes on microscopes to cameras. The "C" is said to stand for "cine", the original application being movie camera lenses, such as were made by Arri, Bolex, Angenieux, Bell & Howell, and Eclair. This standard is both an optical and mechanical specification for the optical source and receiver. The optical source side consists of a tube concentric with the optical axis, ending in a 1"-32 male thread, projecting from a larger flange perpendicular to the axis. The inside diameter of this tube can vary, but practically speaking, an ID of about 0.9 inches is an upper limit to provide enough metal beneath the threads for mechanical strength in the tube. The projection of the male threaded portion from the flange is 4mm (0.157"), and the depth of female threading is 4.5mm (0.177"). Many cameras accept up to 8mm for the threaded portion instead of just 4mm, since 4mm is only about 5 threads of a 32 tpi pitch, and at least 2 of these 5 threads must be relieved for runout up to the shoulder in single-point threading on a lathe, leaving only 2 or 3 fully engaged threads between the components. The light rays of the optical source form an image plane 0.69 inch (17.526mm) away (C-mount) or 12.52mm (CS-mount) from this flange (called the "flange-back" distance), with the circular image field being about 18mm in diameter. The optical receiver consists of a 1"-32 female thread, with a detector and/or further optics based on the image location. A CS-mount camera can work on a C-mount source by simply moving the camera out an extra 5mm with a male/female-threaded spacing ring. Vice versa, a C-mount camera is too "nearsighted" for a CS-mount source, without additional optics.
36e !je q B 5cF_ C-mount "relay lenses" are short optical tubes with male and female C-mount threaded fittings, and optical elements to shrink the image field size. They are specified in terms of power, which ranges from 1.0X (no change in size, just an extension), to fractions such as 0.63X or 0.5X. To apply a relay lens, one chooses the power to shrink the diameter of the C-mount image field to fit the image sensor. For example, a typical image field diameter of 1/2" is much larger than a 1/3" CCD sensor (6mm diagonal), so one could insert a 0.5X relay lens to shrink the image to 6.3mm, so that the CCD pixels would almost complete span the image field.
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