In digital photography ISO refers to an image sensor's sensitivity to light. The lower the number, the less sensitive the sensor is, therefore needing more light for a well exposed image. As you increase the ISO number, your images will appear brighter as the sensor is becoming more sensitive to the light that is landing on it. Traditionally, ISO comes from analogue film photography, it is often seen as “ASA” but it is the same as ISO, just a different acronym.

Films would often be marketed with their recommended use on the box; there would be films called “daylight film” which have an ISO of around 100 or 200, because in daylight you don’t need as much sensitivity. It was advised that a film with an ASA/ISO of 1600 would be used in the evening or at night. A disadvantage of using a higher ISO is that it increases the amount of grain present in your images. As a general rule of thumb, keep your ISO as low as you can, the standard ISO is thought to be 100, which will result in crisp images with little grain.

ISO is one of the three elements in the Exposure Triangle, along with Aperture and Shutter Speed.