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What Is ISO in Photography?

Kate Miller-Wilson
women looking at digital camera

As you begin to control the settings of your camera, one of the first things you'll encounter is ISO. Basically, ISO refers to the sensitivity of your camera sensor or film to the light that hits it through the lens of your camera. It's one of the three points of the exposure triangle, and understanding its role is essential to improving your photography.

Understanding the Role of ISO

Back in the days when photographers only shot film, you could buy film in different speeds, such as 100, 200, 400, and 800. This speed was the film's ISO and told the photographer how sensitive that film was to light. Lower numbers meant lower light sensitivity. A film with an ISO of 100 had to be used in fairly bright light because it was not as light-sensitive as a film with an ISO of 800.

This still holds true in today's digital photography world. Digital cameras use sensors to record light, and photographers have control over how light-sensitive those sensors are. You can set your camera's ISO to 100 for brighter situations or use much higher numbers, often 6400 and above, for low light.

How to Use ISO in Shooting

While you may know ISO makes your sensor or film more sensitive to light, it's a little more challenging to see how this affects your photos. Basically, ISO, shutter speed, and aperture are like three points on a triangle. As you increase the ISO, you are requiring less light to reach the sensor or film in order to create an image. This affects your other settings as follows:

  • Higher ISO means you can use faster shutter speeds, since you don't need to keep the shutter open as long to let in light.
  • Lower ISO means you can use a slower shutter speed to blur motion or to keep too much light from getting in on a bright day.
  • Raising the ISO lets you close the aperture of your lens to let in less light, giving you greater depth of field. This means more things in your photo will be in focus.
  • Lowering the ISO lets you open the aperture of your lens to let in more light, giving you shallower depth of field and allowing you to focus on one thing while letting everything else be blurred.
    exposure triangle

Changing the ISO also gives you more shooting flexibility. On a bright day, you can use a lower ISO and keep too much light from overexposing your image. In a darker situation, such as a child blowing out the candles on a birthday cake, you can increase the ISO and still get the shot.

Sensitivity at a Cost

Besides changing the sensitivity to light, ISO also has an effect on the quality of the image. In the case of film, many of the films with higher ISOs, such as Ilford Delta 3200, had a more prominent grain pattern. This isn't necessarily negative when it comes to film, but it's an important consideration when shooting.

In digital work, a higher ISO setting will result in more noise, which is the digital equivalents of grain. You may have noticed this when taking a cell phone photo in a dark room. In some cases, taking that shot may be worth the noise. Each camera is different in terms of how high you can push the ISO and still have a usable file; it depends very much on the sensor in the camera and how much you enlarge the image.

  • Some cell phones and simple point-and-shoot models can handle ISO up to 800 and then start to have more noise.
  • Other cameras, such as crop-sensor DSLRS use high quality sensors smaller than more expensive models, and they can top out at ISO 1600 or ISO 2000 for usable files.
  • Full-frame DSLRs, such as those used by many advanced amateur and professional photographers, can produce useable files at ISO 6400 and beyond.

Play Around With Your Camera Settings

Ultimately, understanding ISO comes down to practice. Take your camera out of auto mode and take some time to play around with it. Take lots of shots at lots of different settings. You'll learn what your camera sensor can handle when it comes to high ISO, and you'll get a very good sense of how this important variable affects the exposure in your photos.

What Is ISO in Photography?