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The role of the shutter in a camera is to limit the amount of light getting in. Certainly too much light will overexpose film and overwhelm image sensors, but constraining the quantity of light isn't the only function of the shutter. After all, reducing the aperture will similarly prevent overexposure. The shutter controls exposure by its duration, the period during which light is allowed into the camera. Images often change over time—people blink, horses gallop, racecars race, grass grows, and paint peels. By limiting the time during which light is gathered, the shutter can capture a small slice of the image during which movement is minimal, even invisible. A fast shutter can simply stop motion—more correctly, stop the blurring effects of motion on captured images.
Over the years, shutter speeds have been standardized in a sequence in which each speed is one-half the next fastest, effectively cutting the light that passes through by one half. Consequently, each step in shutter speed alters the amount of light reaching the film or image sensor by the same amount as the change in one standard f/stop. Here's a list of the standard shutter speeds:
Cameras differ in the speeds they make available to you. Only more expensive, professional-grade cameras offer the highest speeds, 1/2000th second and greater.
Choosing a shutter speed is not arbitrary. You (or the program of an automatic camera) must consider several issues. The two most important are proper exposure and the minimization of the effect of camera shake.
With 35 mm cameras, the oft-stated rule for the slowest shutter speed that will yield sharp images when the camera is handheld is one over the focal length of the lens (or shorter). For example, with a standard 50 mm lens the slowest speed recommended for handheld exposures is 1/60th second. Some people are able to make sharp exposures at speeds a notch or two slower than the general rule. But when you need to make an exposure longer than these recommendations, you should mount your camera on a tripod to prevent shake. A tripod is not a cure-all, however. Even a tripod won't prevent the image blurring because of subject movement.
Mechanical cameras don't take a picture the instant you push the shutter release. All sorts of cams, gears, and levers swing into action as soon as you press, but light cannot peep through the shutter for several milliseconds as everything comes up to speed. This slight wait is called shutter lag. With a digital camera, there's no mechanical inertia to slow the taking of a picture, but most digital cameras impose their own shutter delay penalties that are substantially longer than those of mechanical cameras. With some digital cameras, you may wait more than half a second between the instant you press the button and when the camera actually makes an exposure. If you don't properly anticipate the action, you may be as surprised as your images. Between the press and the exposure, smiles of surprise melt into shock and anger, high divers disappear into the deep, UFOs accelerate into the ether, and Bigfoot shuffles back into the wilderness. Strangely, longer delays arise most often with more expensive digital cameras.
The cause of the lag is twofold: mechanical and electronic. Even though digital cameras take electronic pictures, they are still dependent on mechanical processes for the optics and focusing. The auto-focusing systems of many digital cameras, especially higher-end cameras that do a better job of it, may require several hundred milliseconds to lock on a subject. You can often eliminate this part of the lag by prefocusing or by focusing manually. The electronic delays are imposed in setting up the electronics to actually capture the image. The time required often increases with the resolution of the camera because there's more to do with more pixels to capture. You can't do anything about the electronic lag except learn to live with it and anticipate action. Newer digital cameras are notably faster than their forebears, but the lag can still be bothersome. Avoid surprises and check the shutter lag of any digital camera before you buy it.
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