In Digital Photography, ISO measures light sensitivity.

In dim light, the shutter automatically opens for longer to let in more light, but this is bad for action photography because it causes motion blur because the shutter remains open too long to freeze the motion.

Flash can be used to alleviate this problem, but for portraits, it is not great because it does not produce a particularly flattering image, and if taking photos of action in the distance, it will not reach far enough so, what do you do?

The answer is ISO.

ISO measures the light sensitivity of the image sensor.

A low ISO setting produces a shot that has finer grain, but the camera sensor is also less light sensitive, while a higher ISO produces a fast shutter speed, and higher light sensitivity. This is why high ISO is used for indoor, or low light sports photography, but the grain of the image is courser.

So, you can get clear, fast shots in low light, what more could you want?

Unfortunately, like many things in life, this comes at a price. Noise.

Not the sound of children shouting, or music blaring from a moody teenagers bedroom. That’s audio noise, here I am talking visual, or image noise.

Noise is also known as grain, which is always present, but when the sensor is at its most sensitive, noise makes an appearance more than ever. Noise is best seen on consistent colours, and shades, such as the sky in the image above, which was taken at full zoom, where distortion can kick in, maximum ISO on a sunny, blue afternoon, but noise has removed detail, and rendered the blue sky to nothing but a greyish mess.

Noise can appear simply as grain, or  cases as coloured blotches, sometimes a vivid blue, purple, green or yellow.

Camera apply noise reduction, but this can soften the image, and cannot remove all noise.

You can view the other two principles of the exposure triangle here: aperture and shutterspeed.