The term ISO refers to how sensitive a digital camera is to light. The ISO control changes how sensitive the sensor is for a shot.
Why do we need it?
A camera can be used in various levels of light. But, it cannot make an image with the brightest light and darkest dark together. We adjust the image sensor to suit the light levels. In bright light we use low sensitivity (ISO 100). In low light we use higher ISO numbers. This makes it more sensitive. In the past film worked the same way. Film speeds were used to match the ambient light levels.
Use of the ISO setting lets us adjust other settings. Bright light means you can use a low number like ISO 100. The camera collects light data more slowly. This means you can use fast shutter speeds and still get enough data. With a high ISO number the sensor collects light data faster. This means a high shutter speed in lower light levels.
The ISO setting
The scale which underlies the ISO definition is called the ‘Exposure Index’ (EI). It relates to image brightness, it’s colour and image appearance. It also includes a digital noise rating for the sensor.
There are various methods to work out the EI. The ISO definition allows manufacturers to choose how to calibrate cameras. The choices reflect sensor tests showing which EI values make well exposed images.
The ISO number
The ISO setting of a DSLR is a number. It shows where on the EI scale the the shot is taken. ISO 100 on many cameras is a high quality rating. Top of the scale ISO may exceed 204800 (reported by Canon and Nikon – Seen online: 06/11/2013). This value far exceeds the ISO of film.
As the ISO number goes up, image quality declines. The image is affected by digital noise. This is a grainy effect on the image. In recent years anti-noise software has reduced noise levels. Modern sensors have also reduced noise.
The International Standards Organisation ISO definition
The detailed ISO definition is set in the “ISO 12232:2006” Standard. You can buy it from the International Organization for Standardization . See: ISO 12232:2006 :: Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index. .
The current ISO Standard was published with corrections in October 2006. It incorporates the older Din and ASA Standards. These were devised for film speeds. The 2006 version is very similar to the previous ones. Users of Din or ASA film will recognise the same scale.