T (for time) keeps the shutter open (once the shutter-release button had been depressed) until the shutter release is pressed again. B (for bulb) keeps the shutter open as long as the shutter release is held. With this scale, each increment roughly doubles the amount of light (longer time) or halves it (shorter time).Ĭamera shutters often include one or two other settings for making very long exposures: The agreed standards for shutter speeds are: Īn extended exposure can also allow photographers to catch brief flashes of light, as seen here. Soon this problem resulted in a solution consisting in the adoption of a standardized way of choosing aperture so that each major step exactly doubled or halved the amount of light entering the camera ( f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.), a standardized 2:1 scale was adopted for shutter speed so that opening one aperture stop and reducing the amount of time of the shutter speed by one step resulted in the identical exposure. In early days of photography, available shutter speeds were not standardized, though a typical sequence might have been 1⁄ 10 s, 1⁄ 25 s, 1⁄ 50 s, 1⁄ 100 s, 1⁄ 200 s and 1⁄ 500 s neither were apertures or film sensitivity (at least 3 different national standards existed). Short exposure times are sometimes called "fast", and long exposure times "slow".Īdjustments to the aperture need to be compensated by changes of the shutter speed to keep the same (right) exposure. Very long shutter speeds are used to intentionally blur a moving subject for effect. Very short shutter speeds can be used to freeze fast-moving subjects, for example at sporting events. In addition to its effect on exposure, the shutter speed changes the way movement appears in photographs. A shutter speed of 1⁄ 50 s with an f/4 aperture gives the same exposure value as a 1⁄ 100 s shutter speed with an f/2.8 aperture, and also the same exposure value as a 1⁄ 200 s shutter speed with an f/2 aperture, or 1⁄ 25 s at f/5.6. For example, f/8 lets 4 times more light into the camera as f/16 does. 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. According to exposure value formula, doubling the exposure time doubles the amount of light (subtracts 1 EV). 
Multiple combinations of shutter speed and f-number can give the same exposure value (E.V.). 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"). This will achieve a good exposure when all the details of the scene are legible on the photograph.
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. Exposure value (EV) is a quantity that accounts for the shutter speed and the f-number. The camera's shutter speed, the lens's aperture or f-stop, and the scene's luminance together determine the amount of light that reaches the film or sensor (the exposure).
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