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The length of time a shutter remains open to allow light to reach the CCD is called shutter speed. The shutter is an electronic control that governs the time taken for the incoming light" to form a single video field. (Each video image, or 'frame' consists of two fields. Since video is recorded at 50 fields per second, that is, 25 frames/second in the Indian video/broadcast system the normal camcorder shutter speed is 1/50 or 1/60 second.) The word 'shutter' is actually borrowed from still cameras where the rate at which light enters the camera is controlled by means of mechanical shutters. Video cameras do not have physical shutters. The shutter in a digital camera is a thin sheet covering the cqp. When this shutter opens, it exposes light from the exterior onto the CCD, allowing a stream of images to be taken.
The key concept here is, the longer the shutter remains open (i.e., the lower the shutter speed), the greater the amount of light that is allowed into the camera, and the faster the shutter closes (i.e., the higher the shutter speed), the smaller the amount of light that is allowed into the camera.
Shutter speed is measured in seconds or fractions of seconds. Using very fast shutter speeds will capture fast-moving subjects, such as race cars clearly. Slow shutter speeds are used to intentionally capture the movement of a subject. How an image is exposed is determined by the combination of the lens aperture and shutter speed. A fast shutter speed will use a larger aperture to avoid an under-exposed image. A slow shutter speed requires a small aperture to avoid over-exposure. Typical shutter speeds are 1/3, 1/6, 1/12, 1/25, 1/50, 1/60, 1/100, 1/120, 1/150, 1/215, 1/300, 1/425, 1/600, 1/1000, 1/1250, 1/1750, 1/2500, 1/3500, 1/6000, 1/10,000 (of a second). Therefore 1/3 is very slow shutter speed whereas 1/250 is very high shutter speed.
Using slower shutter speeds than required can result in blurred images.
Slow shutter speeds can result in a good effect, particularly when shooting traffic at night. The shot will have lights of all vehicles in one stream, with static objects looking good. However, the iris must be open appropriately to get such an effect. Faster shutter speeds are helpful when shooting objects that are moving very fast; for example, a fan.
A tripod should be used when taking long exposures to prevent camera shake. In fact, for shutter speeds below 1/60, using a tripod is recommended to get a steady frame. Figures 2.8(a) and (b) show how the same subject, when shot with shutter speeds of 1/50 and 1/250, with varying exposures, produces different effects.
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