ISO is one of three important settings on your camera that is used to take a well exposed photo. The other two are Aperture and Shutter Speed.
If you’d like to learn about these check out our introduction to aperture and our beginner’s guide to shutter speed.
An Introduction to ISO Settings in Photography
We regularly get questions about ISO from readers of Digital Photography School like these:
What is ISO and why is it important? What is the best setting to choose? Should I always choose the lowest one?’
In this short tutorial I want to answer each question in turn. Let’s start with a definition of ISO.
What is ISO?
ISO in Traditional/Film Photography
In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.
ISO in Digital Photography
In Digital Photography ISO measures the sensitivity of the image sensor.
The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.