A unit of relative exposure used in photography. In optics, a "stop" is a ring-shaped baffle or shield used to reduce the aperture (the diameter of the opening) of a lens. In photography, the amount of light used to record the image can be controlled either by varying the aperture, by varying the length of time the shutter is open to admit light, or by some combination of these two methods. Two exposures differ by one stop if one is made with twice the light of the other; similarly, they differ by n stops if one is made with 2ⁿ times the light of the other. Since the light transmitted varies with the square of the aperture of the lens, narrowing ("stopping down") the aperture by one stop multiplies the f ratio by the square root of 2. On many cameras, aperture settings are provided, or marked on a continuous scale, with f ratios ("f stops") at successive powers of the square root of 2