Slow shutter speeds can be used to create special effects by blurring objects and lights.

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When you press the button on a camera and hear that distinctive click, you're hearing the sound of the camera's shutter opening and closing in a fraction of a second. The shutter opens to let light through to the film or electronic sensor, and closes again. If you alter the length of time the shutter stays open, you can drastically change the kind of image you'll end up with. Along with camera lenses, ISO level, focus and aperture settings, shutter speed is one of the primary factors in photo quality.

There are two important things to remember about shutter speed. First, at slow shutter speeds -- when the shutter stays open for a longer time -- objects in motion will appear blurry in the photograph (and everything will get blurry if the camera itself isn't held still). That's because the same object is recorded in multiple places on the film or sensor as the object moves across the frame during the time that the shutter is open. Second, the longer the shutter is open, the more light reaches the film/sensor, resulting in a brighter overall image.

Shutter speeds are indicated by fractions of a second, with 1/1000 being the fastest shutter speed that you'll find on a typical consumer point-and-shoot camera. Some high-end SLR cameras can shoot as fast as 1/8000 or even 1/16000 of a second. Shutter speeds are standardized so that changing the speed will give you either one half or twice as much light (depending on which way you changed it). For example, changing from 1/30 to 1/60 results in half as much light hitting the sensor.

To change your shutter settings, you turn a dial or switch on an older camera. Today's digital cameras usually feature the shutter speed adjustment via an on-screen menu.

Oh, and that shutter sound we mentioned earlier? If you have a small digital camera, there's a good chance you won't hear the shutter's click at all. You might be hearing a fake electronic shutter sound, because pocket-sized digital cameras use electronic shutters. Instead of a physical shutter opening and closing mechanically, the sensor simply turns itself on and off for the appropriate length of time. Why don't all cameras use this method? The electronics involved force a compromise in image quality, which is why higher-end digital cameras still use mechanical shutters.

What shutter speed is best for which kind of photo? And what special effects can you create by manipulating shutter speed? We'll explain that next.