What is ISO? What is ISO? A camera's sensitivity to light explained, and how to use it?

In photography, making sense of some of the jargon can be a daunting task. That said, it’s key to knowing your way around your camera and taking excellent photos. One of the most confusing settings is ISO, which is one of the three factors of exposure along with shutter speed and aperture. Fortunately for you, we’ve put together this simple explanation to help you better understand what happens when you change your camera’s ISO and how it will affect your images.

In photography, making sense of some of the jargon can be a daunting task. That said, it’s key to knowing your way around your camera and taking excellent photos. One of the most confusing settings is ISO, which is one of the three factors of exposure along with shutter speed and aperture. Fortunately for you, we’ve put together this simple explanation to help you better understand what happens when you change your camera’s ISO and how it will affect your images.

Add caption

First, let’s break down the initialism. ISO simply stands for the International Organization of Standardization, which is the main governing body that standardizes sensitivity ratings for camera sensors (among many other things). The term was carried over from film, when the ISO rating was known as the “film speed” and “ASA.” Having a standard of sensitivity is important, as it allows you to shoot the same ISO on different cameras and trust that the exposure value will be equal.

With both analog and digital cameras, ISO refers to the same thing: the light sensitivity of either the film or imaging sensor. When you change the ISO on a digital camera, you’re rendering the sensor more or less sensitive to light. One of the greatest features of digital cameras — one that is often taken for granted — is their ability to change ISO on the fly. In the days of film, you were essentially stuck with whatever film speed you loaded into your camera, at least until you finished that particular roll.

Digital cameras typically have ISO settings that range between 100 (low sensitivity) to 12,800 or greater (high sensitivity). Compact cameras and camera phones may top out below that, however, and some interchangeable lens cameras with larger sensors go even higher, reaching ISO levels in the hundreds of thousands.

Like shutter speed and aperture, ISO values correspond to exposure “stops,” with an increase of one stop being a doubling of the sensitivity. The relationship of ISO value to exposure stops is very straightforward: ISO 200 is a one-stop increase (doubling of sensitivity) over ISO 100. ISO 6,400 is six stops above ISO 100. If you increase ISO from 100 to 400, you need to balance that with a two-stop decrease elsewhere to maintain the same overall exposure value, say by changing the shutter speed from 1/125 second to 1/500. Obviously, the exact numbers depend on the specific lighting of the situation.

Beyond simply changing the exposure, ISO also affects image quality. A higher ISO typically translates to a noisy or “grainy” image, just as a higher ISO film has more grain than one with a lower ISO. The picture below showcases how ISO can directly impact image quality, though, keep in mind that the particular noise levels depend on the camera.