Click to see the effect of noise in an image.
Click to see the effects of increasing ISO.
Noise appears in images as random color pixels especially when you use long shutter speeds or high ISO settings.
You normally change the ISO using a dial or menu.
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.
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 set 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. The more you increase sensitivity, the more noise you get. This is 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. Some allow you to turn this mode on and off.
During a normal exposure the drip, drip, drip of noise is overwhelmed by the
strong light from the scene. There isn't time for noise to build up in the image.
During a long exposure the noise and the light from the scene are more equal and before the image is fully exposed, noise has time to accumulate.