If you peer inside a camera lens, you will see located within it the all important diaphragm; a device for controlling the amount of light passing through the lens system. This is constructed from a series of flat, interleaving metal blades, which together from a central hole of adjustable size. By turning a ring on the lens barrel, you can change the area of this aperture and, hence, the brightness of the image the TV camera tube receives from the scene.