The shutter speed determines how long the shutter remains open to record the scene. Shutter speeds are measured in fractions of a second or, when shooting a long exposure, in seconds. If you're shooting in dim conditions without a flash, you'll need a slow shutter speed such as 1/15 second. The rule of thumb for the slowest shutter speed at which you can handhold a camera is the reciprocal of the lens focal length. In other words, if you're shooting an image with a digital SLR using a 20mm lens, you'll need a tripod with any shutter speed slower than 1/20 second. You can, however, use a slow shutter speed to create artistic blurs such as the headlight patterns shown in the following illustration. Slow shutter speeds are also useful when photographing a stationary subject in dim lighting conditions. If you're photographing a subject with a slow shutter speed, your subject must remain perfectly still; otherwise, the image will be blurred. When you shoot at a high shutter speed such as 1/2000 second, you freeze action. To avoid blurry images when you're using a high-end digital camera with a zoom lens, you'll need to shoot with a faster shutter speed when you zoom in tight on a subject. When in doubt, a tripod will always ensure that the camera remains steady while you're taking the picture.
You use a slow shutter speed to record images in dim lighting without a flash.
- http://tech.yahoo.com/gd/choosing-the-right-shutter-speed-for-your-digital-camera/191716;_ylt=ApVjhNOy7Q2botqLH15LLRkvLZA5
