What Is ISO and How Do I Use It?

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A primer on ISO sensitivity, and how you can use it to take better pictures

The ISO setting controls how sensitive your camera is to light. Along with shutter speed and aperture, ISO affects exposure—whether a photo looks too dark, too bright, or just right. It also directly impacts image noise—that is, how smooth or grainy a shot will be.

Adjusting the ISO setting is arguably the easiest way to take better-looking pictures (at least as much as any camera setting can help). And just about any dedicated camera, whether it’s a point-and-shoot or a DSLR, will let photographers control ISO (the same can’t always be said for shutter speed or aperture).

This is meant to be a quick, functional guide for casual photographers who want to venture away from auto mode and start to exercise control over their cameras. For a detailed guide on low-light photography—a topic very closely tied to ISO, check out our own guide on the topic. For more detailed, technical explanations of the topics discussed here, we recommend this helpful guide to exposure from Cambridge in Colour, this excellent blackboard-style video about ISO and noise by Dylan Bennett, and this super in-depth explanation of noise by the creators of Imatest.

Why Does ISO Matter?

In a nutshell: Where light is plentiful—outdoors during the daytime—use a low ISO setting. Where there’s less light—indoors or at night—use a higher ISO setting.

Always use the lowest ISO setting that you can get away with.
It’s a good rule of thumb to always use the lowest ISO setting that you can get away with. Picture quality is always better at lower ISO settings. As ISO increases, so does image noise—the ugly, grainy quality you typically see in low-light photos taken with point-and-shoot cameras and cameraphones. Higher ISO settings tend to wash out colors, too.

So if higher ISO hurts photo quality, why would anyone ever increase it? Well, if they didn’t, low-light photos would always be blurry. As the ISO increases, the sensor becomes more sensitive to light, so the camera can use a faster shutter speed and still achieve the same image brightness and exposure—without streaky, blurry subjects in the shot.

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When you double your camera's ISO sensitivity, the shutter speed doubles, too. Here, it jumps from 1/125 to 1/250 of a second as the ISO jumps from 800 to 1600.

How Do I Use ISO Settings in the Real World?

The auto modes on most cameras can automatically pick a suitable shutter speed and ISO setting to take a blur-free shot, even in low light. But learning how to manually adjust ISO gives you some control over how clear or how grainy some of your shots will turn out. It’s an easy concept to understand, and can make the difference between a blurry shot and a crisp one.

A savvy ISO adjustment can make the difference between a blurry shot and a crisp one.
Some cameras have a dedicated ISO button, while others bury the controls in the menu, but it’s almost always somewhere in the system. If you have trouble finding it, try switching to Program (P) mode. (Most cameraphones don’t let users adjust ISO manually—at least not by default. Some photo apps unlock the capability.) Play around with the settings, and take note of how the different settings affect picture quality.

Let’s take a look at some real-world situations where a savvy ISO adjustment can turn a good photo into a great photo.

Say you’re out at a restaurant or bar. Some friends are seated around a table, mostly sitting still. The lighting is dim but warm, and you can easily see everybody’s faces. In auto mode, your camera will probably pick a relatively fast shutter speed, and a relatively high ISO setting—cameras are almost always conservative with shutter speeds, but aggressive with ISO levels. (It’ll also try to convince you to use the flash, but we’re assuming for the sake of this example that you don’t want to use it.) The resulting photo will be mostly blur-free, but probably pretty grainy.

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In this example, the subject is sitting still. So even in the dim lighting of the restaurant, the photo on the left (shot at ISO 800) is blur-free, with better color and significantly less noise than the image on the right (at ISO 6400). The extra ISO sensitivity provided no real benefit in this scenario.

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In this closer crop, the noise in shot on the right (ISO 6400) is even more apparent—and distracting. This is why it's a good idea to shoot at the lowest ISO setting you can manage, without underexposing the shot or risking blur.

To get a better photo, try cutting the ISO setting in half—if the camera picks ISO 1600, drop it down to 800—and shoot the picture again. The shutter speed will only be half as fast, but because your friends are sitting still, motion blur still won’t be a problem. And grainy image noise won’t be as noticeable, either. (Try leaning on a table or against a wall to make your hands extra-stable.)

What about a situation where a higher ISO speed is better? Picture yourself in that same bar or restaurant, but this time, you’re watching a band. The lighting is just as dim, but the musicians onstage are moving around a lot more than your friends at the table were.

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The photo on the left uses a lower ISO setting, so the shutter had to stay open longer to achieve proper brightness—creating motion blur in the process. On the right, the high ISO setting allowed for a quick shutter speed, and a crisper—but grainier—shot of the action.

Most cameras, by default, will choose an ISO setting too low to freeze motion in this lighting. But if you want a sharp, blur-free photo of the guitarist or drummer, you’ll have to crank up the sensitivity. (The photos will be super grainy, though.)

High-ISO image noise varies significantly from camera to camera—it’s always one of the biggest topics of discussion in both professional and user reviews of cameras. So it’s tough to recommend specific settings for specific cameras. You’ll have to experiment with your particular camera to find a high-ISO ceiling that works for you.

It's fair to say, however, that smaller sensors take worse pictures than big sensors at high ISO settings. So with point-and-shoots (including superzooms), it’s best to keep the ISO setting at 800 or below, though ISO 1600 is sometimes acceptable these days. Advanced compacts, which have larger-than-average sensors, generally look OK at ISO 1600. Interchangeable-lens cameras look great at ISO 1600, and can increasingly handle ISO 3200. The best models even look pretty clean at ISO 6400 and higher.

ISO sensitivity is a major piece of the puzzle of photography. Once you've put in some practice and grasped the concept, you'll notice the quality of your photos improve, and the rest of the manual controls on your camera won't seem so intimidating. It's an important, but approachable step toward becoming a more capable photographer.