What is ISO

ISO

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.

Higher numbers mean your sensor becomes more sensitive to light which allows you to use your camera in darker situations. The cost of doing so is more grain (although cameras are improving all the time and today many are able to use high ISO settings and still get very useable images).

An example of a situation you might want to choose a higher ISO would be photographing an indoor sporting event where the light is low and your subject is moving fast. By choosing a higher ISO you can use a faster shutter speed to freeze the movement.

ISO Settings and Grain

As mentioned – the cost of choosing higher ISO settings is that you begin to get higher grain or noise in your images the higher you go.

I’ll illustrate this below with two enlargements of shots that I just took – the one on the left is taken at 100 ISO and the one of the right at 3200 ISO.

100 ISO is generally accepted as a ‘normal’ or ‘standard’ ISO and will give you lovely crisp shots (with little to no noise/grain).

Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also.

When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example – if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.

ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. Experiment with different settings and how they impact your images today – particularly learn more about Aperture and Shutter Speed which with ISO are a part of the Exposure Triangle.

How to use ISO sensitivity when shooting video

The Video Mode takes a look at the how best to use ISO sensitivity when shooting video, and how it compares to taking stills. There are plenty of similarities in ISO settings between the two.
For example, sticking with lower ISO sensitivity between a range of ISO 100 – 400 will generally producer higher quality results than shooting with ISO 1600 and above.
That’s because the ISO control regulates the sensitivity of the sensor’s pixels to light by boosting the electrical charge. A higher ISO creates a brighter image but because of the increased electrical signal, the recorded images or video will be covered in digital artefacts, sometimes referred to as “grain” or “noise”.

Using ISO sensitivity: Grain

For some projects, grain can be used to good effect to create a more cinematic or artistic looking video. But as you become more experienced in video production you’ll learn that, while as photographers, we aim to get everything as close to perfect in-camera, in filmmaking, you’ll want as clean an image as possible with the maximum amount data to work with in post-production.
If you’ve recorded clean footage with low noise, you can add the precise amount that you want during the editing process.

Using ISO sensitivity: Noise

It’s good to experiment with ISO settings to find what you feel is an acceptable level of noise for the videos you’re making. There’s also an interesting quirk with some cameras, mostly Canon branded, that suggests that using ISO sensitivities in multiples of ISO 160 to reduce the impact of noise.
In short, the theory is that using these interval ISO settings decreases the impact of noise at the cost of reduced dynamic range. For more information, read this insightful discussion as to why you may get better results if you decide to use 160-multiples.
Newer cameras are much better equipped at dealing with high ISO sensitivity settings though because they use clever algorithms to reduce the impact of noise.
Cameras with larger sensors and cameras designed for video such as the Canon Cine EOS also handle noise better because they have larger pixels. But all things being equal, the lower the ISO, the better the quality of the recorded image.