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What is ISO? Making sense of sensitivity in photography

(Image credit: Future)

Back in the days before digital, film came in a variety of different speeds. The ‘faster’ the film, the more sensitive it was to light – allowing you to use faster shutter speeds than with ‘slower’ film. Using these higher-sensitivity film emulsions was useful for moving subjects – and particularly so in low light. This film speed was measured using a number of different scales – with two of the best known, the American ASA and German DIN scales, eventually being brought together to give us the standardized ISO system.

Digital cameras, of course, do not use film – but the same ISO scale is now used to measure the camera’s sensitivity to light. Although the camera’s imaging chip cannot be changed to suit the subject (unlike film), its sensitivity can effectively be boosted by the camera’s circuitry. This is done with the ISO control. 

Think of ISO as being like the volume control on your radio. If the signal is weak, you crank it up to compensate. The signal from the sensor is simply amplified – and this helps you get the fast enough shutter speed you want in low light. 

See also: What is exposure?

Turning up the ISO is like turning up the volume - and the higher the go the more distortion or noise you get (Image credit: Future)

What does ISO stand for?

The advantage of digital over film is that the ISO can be altered for each individual shot. This makes ISO a powerful tool for the photographer, helping you to get sharp shots in a variety of lighting conditions. 

ISO is the name of the International Organization of Standardization: a body that creates thousands of agreed standards for a huge range of products, procedures and practices. For the photographer, ISO is simply a set of numbers. 

The base sensitivity of most digital cameras is ISO100. But is typically increased by pressing the appropriate button then rotating a dial – or by using a menu setting. On some cameras you may even get a separate ISO control dial. 

The scale is such that doubling the ISO number doubles the sensitivity of the sensor. So increasing the ISO setting from 100 to 200 means that, to get the same overall exposure, you can use a shutter speed that is half as long (or twice as fast). 

Each doubling of the ISO also increases the sensitivity by a full exposure ‘stop’ – with the typical full-stop ISO scale progressing 100, 200, 400, 800, 1600, and so on. The top ISO setting varies depending on the age and cost of your camera. Typical maximum settings range from ISO3200 to ISO819,200.

(Image credit: Future)

Bizarrely, the top ISO settings on some models are ‘hidden’, and must be enabled using a custom option called ‘ISO Expansion’ or similar. The reason for this is that each time you increase the ISO setting, you also get a small decrease in image quality. Boosting the picture signal also amplifies impurities in the signal known as ‘noise’. This noise shows up as grain and color mottling in the image – and this gets progressively more noticeable the higher the ISO is set. 

When to increase the camera's ISO

You only need to use a high ISO in low light if the camera is handheld, or the subject is moving. With static subjects, and a tripod, you can use the slowest setting - here a setting of ISO100

You only need to use a high ISO in low light if the camera is handheld, or the subject is moving. With static subjects, and a tripod, you can use the slowest setting - here a setting of ISO100 (Image credit: Chris George)

For lowlight images where the subject is moving, a higher ISO (here ISO1600) become essential (Image credit: Chris George)

Some photographers try to resist increasing the ISO at all costs in search of getting the best, grain-free images. However, pumping up the ISO often actually increases image quality overall, as this simple change lets you use a faster shutter speed – thereby eliminating camera shake. A grainy picture is always better than a blurry one! A higher ISO can also enable you use a narrower aperture – increasing depth of field, and thus increasing the resolution of a lens – to give you sharper-looking pictures. 

Although higher ISO settings are invaluable in low light, they are not essential for all low-light situations, in fact, if you can keep the camera steady, they are often best avoided. If you are using a solid tripod, the slowest ISO setting (ISO100) is usually the best option – as you can then use a longer shutter speed to make up for the lack of light. Similarly, if you are using flash, high-ISO settings are not needed (although increasing the ISO will increase the effective range of your flash).  

Types of noise

Luminance noise: Traditional monochromatic grain in darker areas (as seen in the wall behind the statue in the detail above right) is typical of luma noise. (Image credit: Chris George/Digital Camera World)

Chromatic noise: Lowlight interior shows chromatic noise in shadows, as seen in the rainbow speckles in the detail (above right) (Image credit: Chris George/Digital Camera World)

There are two different types of noise found in digital images. Luminance noise shows up as a speckled pattern, like specks of black sand, and is similar to the grain that was found when using high-ISO black-and-white films. Chromatic noise is colored and looks like the rainbow-like sheen when looking at a patch of oil (and is similar in appearance to the blotchy dye patterns that you saw when enlarging high-ISO color films). 

It’s important to look at these two types of noise separately – as each can be reduced using different tools during the editing stage. These are often provided as separate noise-reduction sliders by a RAW converter (such as in Adobe Photoshop’s Camera Raw utility). Specialist software, such as DxO Dfine , is particularly useful for reducing noise without sacrificing detail.

Read more:
Photography tips for beginners: 12 things to learn with your camera
Cheat sheet: How to understand ISO settings
Cheat sheet: How to understand f-stops
Cheat sheet: Shutter speed stops