Photography cheat sheet: How to understand ISO settings

Canon camera with ISO settings
(Image credit: Future)

The ability to adjust your camera's sensitivity (ISO) is one of the great advantages of digital capture. Unlike with film cameras (opens in new tab), you can change the ISO for every shot with ease, should you wish to. But when and why should you change this fundamental photographic control, which, along with aperture (opens in new tab) and shutter speed (opens in new tab), makes up part of the exposure triangle (opens in new tab).

But what is ISO (opens in new tab), exactly? Changing your ISO setting means that you can tailor the camera to various situations. Doing so will help you to end up with an image that's sharp and appropriately exposed, whatever the environment.

What ISO setting should you use?

The best low-light cameras (opens in new tab) are more capable of shooting in dim conditions than other cameras, but as a general rule, you want to stick to the lowest ISO possible, as this will give you the cleanest images. 

This is easy in good lighting conditions as your camera doesn't need a very high ISO setting in order to capture the image. So, if there's plenty of light in the scene, you can use a setting such as ISO 100 or 200.

When shooting in trickier conditions, you may need to raise this ISO, perhaps to ISO 3200 or 640000 – or maybe even higher. Doing this will give you a faster shutter speed to work with, which in turn helps you to end up with a sharp image.

Image noise, however, is likely to be more visible. This gives images a colored, grainy texture, and it can obscure details, so you really want to keep this to a minimum.

Our photography cheat sheet below can help you to understand the ISO scale, and when to use different ISO settings for different subjects.

Photography cheat sheet: ISO settings

Click the top-right-hand corner to enlarge this photography cheat sheet image. Why not save it for reference later? (Image credit: Digital Camera World)
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If you're using your camera on a tripod (opens in new tab), you can use a lower ISO setting than you would normally be able to, which will help with the quality of your images. This is because the camera is not subject to any movement during the exposure as it's not being held by the user, which makes longer shutter speeds practical. 

To give an example, an sensitivity of ISO 100 may require a shutter speed of three or four seconds in a particular situation. This is too long to be useful when hand-holding the camera; the result will be a blurry image.

On a tripod, however, you can use this kind of ISO and shutter speed in the knowledge that the camera won't move at all during the exposure. This is great as it allows you to record moving subjects, such as flowing water or traffic, with a pleasing blur.

Today's cameras allow you to select an Auto ISO option. This allows the camera to choose the best ISO for a particular situation, basing its decision on your lens and the shooting conditions, and conveniently alter this from frame to frame. 

There may be times, however, when you want to overrule this, perhaps for creative reasons. So, it's good to understand what's happening. 

Save our cheat sheet and make sure you never get stuck with ISO again!   

We've got many more photography cheat sheets (opens in new tab) for you, on subjects as diverse as woodland photography (opens in new tab) to cityscapes (opens in new tab).

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Chris George has worked on Digital Camera World since its launch in 2017. He has been writing about photography, mobile phones, video making and technology for over 30 years – and has edited numerous magazines including PhotoPlus, N-Photo, Digital Camera, Video Camera, and Professional Photography. 


His first serious camera was the iconic Olympus OM10, with which he won the title of Young Photographer of the Year - long before the advent of autofocus and memory cards. Today he uses a Nikon D800, a Fujifilm X-T1, a Sony A7, and his iPhone 11 Pro.


He has written about technology for countless publications and websites including The Sunday Times Magazine, The Daily Telegraph, Dorling Kindersley, What Cellphone, T3 and Techradar.

With contributions from