ISO, the International Standards Organisation is the group that helped establish a benchmark rating for describing a film's light sensitivity, often called film speed.
Films are therefore differentiated by their speed or ISO number.
Consumer film ranges from 100 to 800, with higher numbers indicating faster - more sensitive - film.
The light sensitivity rating has been redefined for digital cameras, indicating the image sensor' sensitivity to light.
In order to give photographers a better chance of capturing a good image in dim or very bright lighting, most DSLRs have adjustable light sensitivity setting ranging from 100 to 3200 ISO.
The ISO setting is the third variable that determines the amount of light that exposes the image sensor and is key for achieving a correct image exposure . Increasing it to 200 doubles the light sensitivity of the image sensor, while dropping it to 50 halves the light sensitivity of the image sensor.
Like many of the other settings on a digital camera, changing the light sensitivity setting comes at a cost. As you move up the light sensitivity scale, some cameras have difficulty maintaining consistent exposure for every single pixel in the image introducing digital noise in the image.
The digital noise is a polka-dot effect which is most noticeable in images shot in low-light situations. Many consider the digital noise equivalent to grain when using film camera but the effects are quite different.
Some film photographers will purposely shoot images with enhanced grain for artistic effect. However, the digital noise detracts from the image quality and lacks the artistic effect of enlarged film grain.
You can reduce digital noise by taking your photograph at light sensitivity settings between 100 and 400. But note that the amount of digital noise produced when you increase the light sensitivity setting varies from camera to camera. You might want to test how much digital noise your camera creates at different settings.
If your subject is not moving and you cannot use a flash, using a tripod can allow you to shoot successfully at low light sensitivity settings thus reducing digital noise.
Many DSLRs offer a noise reduction feature. This option applies a software filter to erase noise as part of the file processing that occurs as your camera stores the image. The main disadvantage of this feature is the significant lag time required for the image to process between shots.
Often a situation occurs where the lighting is not quite right for your image and this signals the time to increase your ISO setting. Here are some examples:
Shooting in low-light situations, such as early evening and indoors - natural light has a certain appeal as it eliminates harsh shadows and produces more natural colours. By increasing your camera light sensitivity you might be able to shoot your image without using flash at all.
Your subject is too far way for the flash to have an effect - you might encounter a situation during a particularly overcast day or during winter where the light is just not quite enough. You camera wants a flash but your subject is just too far away. So to properly expose your image you need to increase your light sensitivity setting.
Shooting at night - most digital cameras have very limited ability to take images at night. If you want to capture anything at all with a night shot, you will need to increase the camera's light sensitivity.