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In photography, shutter speed or exposure time is the length of time when the film or digital sensor inside the camera is exposed to light, also when a camera's shutter is open when taking a photograph.〔 〕 The amount of light that reaches the film or image sensor is proportional to the exposure time. 1/500th of a second will let half as much light in as 1/250th. ==Introduction== The camera's shutter speed, the lens's aperture (also called f-stop), 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). 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). Moving the aperture ring or changing the aperture setting between the following scale of f-numbers/apertures (f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22...) lets twice as little light into the camera (from left to right on the scale) or twice as much (from right to left. f/8 let 4 times more light into the camera as f/16 (a two-stop difference thence 2x2). A shutter speed of 1/50 s with an 4.0 aperture gives the same exposure value as a 1/100 s shutter speed with an 2.8 aperture, and also the same exposure value as a 1/200 s shutter speed with an 2.0 aperture, or 1/25th second at f/5.6. 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". Adjustments to the aperture need to be compensated by changes of the shutter speed to keep the same (right) 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). 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 (2.8, 4, 5.6, 8, 11, 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. The agreed standards for shutter speeds are: With this scale, each increment roughly doubles the amount of light (longer time) or halves it (shorter time). Camera shutters often include one or two other settings for making very long exposures: * B (for ''bulb'') keeps the shutter open as long as the shutter release is held. * T (for ''time'') keeps the shutter open (once the shutter-release button had been depressed) until the shutter release is pressed again. The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of the slowest possible shutter speed for a handheld camera. The rough guide used by most 35 mm photographers is that the slowest shutter speed that can be used easily without much blur due to camera shake is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 s (closest to "50"), for a 200 mm lens it is recommended not to choose shutter speeds below 1/200th of a second. This rule can be augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and closeup viewing would require faster shutter speeds to avoid obvious blur. Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement, using a monopod or a tripod, slower shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support, usually a tripod, must be used. Image stabilization on digital cameras or lenses can often permit the use of shutter speeds 3–4 stops slower (exposures 8–16 times longer). Shutter priority refers to a shooting mode used in cameras. It allows the photographer to choose a shutter speed setting and allow the camera to decide the correct aperture. This is sometimes referred to as Shutter Speed Priority Auto Exposure, or TV (time value on Canon cameras) mode, S mode on Nikons and most other brands. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Shutter speed」の詳細全文を読む スポンサード リンク
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