Definition: ISO

ISO

In photography the term ISO commonly refers to the standard which is recognised internationally as representing the light sensitivity of a digital Camera. The Standard refers to the camera sensitivity rather than individual components within it. This is because the light inside the camera is affected by other components, the shape, reflectivity and other factors. This in turn affects the amount of incident light interacting with the sensor points on the digital image sensor.

The light sensitivity of a modern digital camera is expressed in a single ISO numerical term. Thus, ISO 100 is seen on most digital SLR cameras the highest quality rating for ISO. At the other end of the scale ISO ratings may exceed ISO 204800. This latter value represents sensitivity to light that far exceeds chemical-based films. However, the penalty for using high ISO ratings is digital noise – a spatter of artefacts or imperfections and defects on the image caused by the sensor. In recent years noise reduction software has significantly reduced the noise levels at high ISO. The improvement in sensor technology has also reduced noise.

Digital Still Cameras (as referred to in the ISO Standard) use an ‘Exposure Index’ (EI) rating which is commonly called the ISO Setting. This is a scale that is specified by the camera manufacturer. It relates to the relationship between the brightness of the output image, the colour of the output image, the standard for colour (sRGB colour-space), appearance of the resulting image and the noise rating from the sensor. There are a number of different methods for determining this Exposure Index under the ISO standard. It is up to manufacturers to make a judgement about how to apply these techniques to calibrate the camera. In practice the choices are an outcome of the manufacturer’s opinion of what EI values produce well-exposed JPEG (sRGB) images at the various sensor sensitivity settings.

The term ISO is an acronym. It stands for (International Organization for Standardization . The Organisation is… “a network of the national standards institutes of 163 countries” .

The standard for modern digital sensors comes under the management of the International Organization for Standardization (ISO) and is recognised by the following reference: ISO 12232:2006. “Photography – Digital still cameras – Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index” (first published in August 1998, revised in April 2006, and corrected in October 2006). This ISO standard combines the older Din and ASA standards which were devised to calibrate the speed of film. The practical numerical standard used to set the ISO is very similar to the previous standards. Users of Din or ASA film will recognise the same numerical calibration.

Related articles/links:
ISO: get control of your sensitive camera!
Definition: Exposure
Definition: Aperture
(International Organization for Standardization 
ISO 12232:2006