Click to see the effects of increasing ISO. Click to see the effect of noise in an image.

In dim light, you or the camera can only get a good exposure by leaving the shutter open longer or increasing the camera's sensitivity to light— called its ISO.

Sensitivity

One way to improve sharpness in dim light is to increase the camera's sensitivity. This works in places such as theaters and gyms where subjects are too far away for flash to work and where you need a faster shutter speed to eliminate blur. It also is a good way to get pictures without using flash in places such as concerts and museums where flash is prohibited. Digital camera sensitivity is usually specified as an ISO setting just as the speed of film is. Increasing the camera's sensitivity or ISO means less light is needed for a picture so you can use a faster shutter speed to freeze action or reduce blur caused by camera movement. Sensitivity on some cameras can be adjusted between 50 and 6400, a range of 8 stops, but most offer a smaller range of settings. The price you pay for using the higher settings is noise randomly spaced bright pixels concentrated in dark areas of the image.

Noise Reduction

Using slow shutter speeds, especially those over one second, or higher ISO settings creates noise in an image. The slower the shutter speed or higher the ISO, the more noise you get. This is partly because long shutter speeds let noise build up and partly because digital cameras increase sensitivity by amplifying the signals captured by the photosites on the sensor— similar to turning up the volume on the radio. Dim light can be made brighter this way but unfortunately, amplifying the image also amplifies noise. Many cameras have a noise reduction mode designed to reduce or eliminate noise caused by long exposures or high ISO settings. Some allow you to turn this mode on and off, or set it to auto so it's used only when necessary.