<P> The camera's shutter speed, the lens's aperture (also called f - stop), the Film Speed (a.k.a. ISO), and the scene's luminance together determine the amount of light that reaches the film or sensor (the exposure). Exposure value (EV) is a quantity that accounts for the shutter speed and the f - number . Once the sensitivity to light of the recording surface (either film or sensor) is set in numbers expressed in "ISOs" (ex: 200 ISO, 400 ISO), the light emitted by the scene photographed can be controlled through aperture and shutter - speed to match the film or sensor sensitivity to light . This will achieve a good exposure when all the details of the scene are legible on the photograph . Too much light let into the camera results in an overly pale image (or "over-exposure") while too little light will result in an overly dark image (or "under - exposure"). </P> <P> Multiple combinations of shutter speed and f - number can give the same exposure value (E.V.). According to exposure value formula, doubling the exposure time doubles the amount of light (subtracts 1 EV). Reducing the aperture size at multiples of one over the square root of two lets half as much light into the camera, usually at a predefined scale of f / 1, f / 1.4, f / 2, f / 2.8, f / 4, f / 5.6, f / 8, f / 11, f / 16, f / 22, and so on . For example, f / 8 lets 4 times more light into the camera as f / 16 does . A shutter speed of ​ ⁄ s with an f / 4 aperture gives the same exposure value as a ​ ⁄ s shutter speed with an f / 2.8 aperture, and also the same exposure value as a ​ ⁄ s shutter speed with an f / 2 aperture, or ​ ⁄ s at f / 5.6 . </P> <P> In addition to its effect on exposure, the shutter speed changes the way movement appears in photographs . Very short shutter speeds can be used to freeze fast - moving subjects, for example at sporting events . Very long shutter speeds are used to intentionally blur a moving subject for effect . Short exposure times are sometimes called "fast", and long exposure times "slow". </P> <P> Adjustments to the aperture need to be compensated by changes of the shutter speed to keep the same (right) exposure . </P>

The time the shutter is open when you take a photo in daylight