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The shutter is the device in a camera which obstructs light to the sensor. When the camera fires, the shutter briefly opens to expose the camera sensor to a controlled amount of light. The shutter then closes, obstructing the light again. Shutter speed is the time the shutter is open. This means it is the length of time a camera's image sensor sees the scene. This is usually a small fraction of a second. Shutter speeds are measured in fractions of seconds. These times range from 1/8000 to several seconds long. Speeds of 1/60 or faster are the most commonly used speeds.  Anything below 1/60 may result in camera shake, which causes blurring on the picture. You will need to use a tripod if you use slower speeds.  Typically only the denominator is marked on the camera. For example, "125" means 1/125 second. Some cameras allow you to take photos at a shutter speed in full seconds, such as 1, 2, or 10 seconds. This is used for low light photography and a lot of movement. To know which shutter speed you should use in a situation, you first have to know what a fast and slow shutter speed is. Generally, 1/60 is the base shutter speed that marks the boundary between fast and slow.  Denominators larger than 60, like 1/125, 1/500, or 1/2000, are fast shutter speeds. Denominators below 60, like 1/30 and 1/15 are slow. Shutter speeds that are full seconds long, like 1 or 2 seconds, are very slow shutter speeds. Most cameras have a shutter priority setting. This mode will allow you to choose the shutter speed based on the picture you want to take while the camera matches the aperture so you get the best exposure.  On most cameras, the shutter priority setting is labeled "S." On some cameras, like Canons, this setting is labeled "Tv." You can shoot in aperture mode and let the camera choose the shutter speed while you set the lens aperture. In manual mode, labeled "M," you set the shutter speed and aperture. The focal length of your lens can cause camera shake. Because of this, you need to take the focal length into consideration when choosing a shutter speed. If you have a long focal length, you will probably want to use a faster shutter speed. The shutter speed denominator should be at least equal, if not larger, than the focal length. For instance, a 50mm lens should be casually handheld at speeds no slower than 1/50 second; a 200mm lens should be no slower than 1/200.
Understand shutter and shutter speed. Know how shutter speed is measured. Learn the difference between fast and slow shutter speeds. Find your shutter priority shooting mode. Think about focal length.