ISO Stands for International Standards Organizations. In photography its a standardised way of measuring sensitivity to light.
With traditional photography it is used to define how sensitive the film is to light. It is measured using numbers typically 100, 200, 400, 800, 1600 etc. Lower numbers mean the film is more sensitive than higher numbers.
In digital photography the image sensor replaces the film. The ISO value now indicates the sensor’s sensitivity to light. Because the image sensor is controlled by software the ISO values can be changed using the camera’s controls. Depending on the type of camera the ISO range will vary.
By selecting a lower value the image sensor is less sensitive to light and produces a finer grain. In situations where there is plenty of light it’s a good idea to keep the ISO setting as low as possible (100 is appropriate for most cameras). This will ensure noise is kept to minimum. However if using very fast shutter speeds such as 1/1000 or greater, images may appear very under exposed (dark). Raising the ISO value will increase sensor sensitivity and produce a correctly exposed shot. Night shots with slow shutter speeds may also require higher ISO values for correct exposure.
High ISO values a generally used in low light conditions to get faster shutter speeds.
100 ISO is generally classified as ‘normal’ and produces a clean, crisp shot.
Many camera’s provide an automatic option for ISO levels. The camera will analyse the current light intensity and attempt to keep the value as low as possible.
By using manual ISO settings you have more control over aperture and shutter speeds. However it can be more difficult to balance ISO, aperture and shutter speed for a well exposed shot.
Selecting the correct ISO involves good assessment of the shot required. You need to take into account lighting levels, whether the subject is moving, are you using a tripod and how much picture noise is acceptable.
It best to keep ISO levels low. However there are situations where a higher level is acceptable such as museums, concerts and indoor sport events.