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What is the Difference Between C & CS Mount lens?

The physical difference is the CS mount lens is designed to be mounted ~5mm closer to the image sensor than a C mount lens. (C-mount lenses are designed to be mounted 17.526mm in front of the image sensor vs. 12.5mm for CS-mount.) You can always use a C mount lens on a CS mount camera by using a 5mm spacer ring (many cameras now have C/CS selectable adjustment screws or rings). You can never use a CS mount lens on an older style C mount camera unless you are willing to physically modify the camera. Cost wise the CS mount lens is much less expensive since it uses fewer glass elements. Quality of image is the same. C mounts are becoming less and less popular and are generally only used on the more telephoto focal lengths such as 25, 50 and 75mm, and bigger zooms.

Both the C and CS mount are 1 inch wide (25.4mm) with 32 threads per inch (0.03125 inches or 0.79375mm). This dimension comes in handy if you need to insert a spacer to obtain proper focus. Unscrew the lens (or unscrew the camera from the mount in the case of telescope use and count the turns until proper focus is obtained. Multiply the above dimension by the number of turns to obtain the needed spacer or washer. (Washers are sometimes used as spacers if there are enough threads available.) Example: 1.25 turns x 0.79 mm = 0.9875 or ~1 mm. Many cameras (especially newer ones) have set screws to allow small adjustments in the distance between the lens and the image sensor.

If you are using a zoom lens and notice that the system goes out of focus as the zoom is changed the distance between the lens and the image sensor needs to be changed. Set the lens for infinity focus. Zoom to wide angle and point the camera at a distant object or star. Adjust the distance between the lens and the image sensor for proper focus. Go to maximum zoom. Slightly adjust the focus ring on the lens to refocus the image if needed. The system should now maintain focus from minimum to maximum zoom. You may need to repeat this process more than once to refine the focus.

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