Most modern security cameras and lenses are generally CS mount rather than C mount. With CS mount cameras both types of lenses can be used but the C mount lens (typically ZOOM lenses still are “C” format) requires a 5mm ring to be fitted between the camera and lens to achieve a focused image. (Also, without this ring you may damage your ccd chip) With C mount cameras it is not possible to use CS mount lenses as it is not physically possible to mount the lens close enough to the CCD to achieve a focused image.

So when I get these calls, usually the quick answer is “take the lens ring off,”  As the imager cannot get close enough to the CCD.