Digital Camera Glossary: ISO

what is ISO
Shooting in low light conditions requires increasing the ISO setting on the camera. Kevin Kozick / Getty Images

You may have noticed an ISO setting on your digital camera. If you're new to digital photography, you probably ignored it, allowing the camera to just shoot at an automatic ISO setting. But as your photography skills advance, you're going to want to learn to control the ISO. And to do that properly, you're going to need to figure out the answer to the question: What is ISO?

Understanding Your Camera's ISO

ISO is a number used to express the light sensitivity of the digital camera's image sensor.

 Higher ISO settings allow you to shoot digital photos in low-light conditions, but such photos are more susceptible to noise and grainy images than photos shot at low ISO settings. Lower ISO settings decrease the sensitivity of the image sensor to light, but they also don't suffer from problems with noise. 

Low ISO settings are best used in outdoor photography, where the lighting is very good. Higher ISO settings are best used in indoor photography, where lighting is poor.

Dating Back to Film Photography

ISO has its origins in film photography, where the ISO setting measured the sensitivity of a particular roll of film to light. Each roll of film would have had a "speed" rating, which was also marked as the ISO, such as ISO 100 or ISO 400.

You'll find that with a digital camera, the ISO numbering system has carried over from film. The lowest ISO setting for most cameras is ISO 100, which was equal to the most commonly used film speed.

Certainly, you'll find ISO settings on a digital camera that are lower than ISO 100, but they'll mainly appear in higher end DSLR cameras. 

What Is ISO and How Do I Set It?

With your digital camera, you usually can shoot at a variety of ISO settings. Look for an ISO setting in the camera's menus, where each ISO setting will be listed numerically, along with an Auto setting.

Just select the number that you want to use for the ISO. Or you can leave the ISO at the Auto setting, and the camera will select the best ISO to use, based on the measurement of the lighting in the scene. 

Some very simple, older point and shoot cameras may not give you the option of setting the ISO yourself, in which case you will not see an ISO setting in the menus. But this is very rare with any newer camera, as even the most basic digital cameras, and even some smartphone cameras, give you the ability to set the ISO manually.

ISO settings usually double as they increase. So you'll see ISO numbers go from 100 to 200 to 400 to 800 and so on. However, some advanced digital cameras, such as some of the best DSLRs, will allow more precise ISO settings, such as going from ISO 100 to 125 to 160 to 200 and so on. The doubling of the ISO number is considered increasing the ISO by one full stop, while the more precise measurements are considered increasing the ISO by one-third of a stop.

Some advanced cameras may even make use of what's called extended ISO, where the highest ISO settings may not be expressed as a number, but instead as High 1 or High 2. There even may be a Low 1 or Low 2.

These extended ISO settings are not recommended by the camera manufacturer to be used, expect under the most extreme circumstances that you may encounter as a photographer. Rather than using the extended ISO setting in a low light photograph, you may want to use a flash.

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