What Exactly Is ISO, Anyway?
Your ISO tells your camera how bright or dark to make your photo, based on the other exposure settings you choose, like aperture and shutter speed. Together, these are known as the “exposure triangle,” but more on that later. You might also hear ISO referred to as the camera sensor’s signal gain, since it’s picking up more light the higher you crank your ISO setting.
Base ISO is the lowest setting your camera comes with from the factory, also called “native ISO.” The lowest to the highest ISO settings your camera has out of the box is called the native ISO range.
The term originally came from the 35mm film days, where film sensitivity or “film speed” measured how sensitive a strip of 35mm film was to light. A strip of film with a higher setting, like 800, is more sensitive and burns faster when exposed. In the film days, a film’s speed was also called its ASA rating. ASA was later adapted to ISO for digital cameras. In the same way, high-ASA film burns faster, a high ISO setting will tell your camera’s sensor to absorb light more quickly.
Raising your camera’s ISO to higher levels like 800 or more is sometimes needed to get the proper exposure. Raising it too high, though, results in digital grain or “noise” in the photograph. This noise looks sort of like static and when it gets bad enough it can make a photo unusable. That said, most modern DSLRs and mirrorless cameras handle high ISO settings very well and you can push them pretty far without noticeable noise.
Your ISO setting will also affect the dynamic range of your photograph. Dynamic range is how much detail gets captured in the brightest and darkest areas of a photo. Shooting at a camera’s base ISO, usually 100, results in the lowest noise and highest dynamic range.
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How ISO Interacts With Other Camera Settings
ISO works with two other settings on your camera when it’s in manual mode to control the brightness or exposure of your photograph: shutter speed and aperture. Your shutter speed is how long the shutter opens to expose the sensor to light, while your lens aperture is the opening your lens uses to let that light in.
Your lens aperture lets in a lot of bright light when opened very wide, and if your camera’s ISO setting is too high that light could blow out your photo. So, you’d turn your ISO down to get a more even exposure. If, for example, you were shooting an outdoor portrait and wanted to use a wide aperture setting, you would need to adjust your ISO to compensate for the extra light.
It’s the same with shutter speed. If you’re capturing fast action and your shutter is only opening up at 1/1000th of a second to let light in, you need your sensor to capture light faster to get a proper exposure. So you’d turn your ISO up.
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How Do I Choose the Right ISO Setting?
Different situations will call for different camera settings. A change in environment, lighting, or subject matter will all affect what ISO it makes sense for you to use. A dimly lit concert venue, for example, will demand a totally different setting than an outdoor field at sunset.
Generally speaking, you’ll want to turn down the ISO for brighter environments and up for darker ones. To avoid as much digital noise as you can, it’s a good idea to keep the ISO as low as possible while still getting an even exposure. Everything when calculating exposure is a tradeoff, and finding the balance will come with experience.
Sometimes you can skirt these rules. If you have a tripod, for example, you can get away with a low ISO setting because the tripod will keep the camera stable long enough to use a slower shutter speed to gather more light. If you have a very wide-aperture lens, you can use a lower ISO setting in dim places and still get enough light for a decent exposure.
RELATED: What Shutter Speed Should I Use With My Camera?
For a good visual rundown of ISO and the exposure triangle in general, check out this video from photographer and videographer Tyler Stalman.
