What Is ISO?

How to take professional photos by changing camera light sensitivity

Image noise visible at high ISO

ISO is a measure of sensitivity to light. In conventional cameras, this is known as film speed, the photographic film’s sensitivity to light. In a digital camera, it relates to the sensitivity of the digital sensor to light.

What Is ISO?

When taking photos in low-light, increasing the ISO setting is one of three ways to get a good quality image; increasing aperture and decreasing shutter speed are the other two ways.

  • Increase ISO: The film or sensor becomes more sensitive, allowing a brighter image without increasing the amount of light entering the camera.
  • Increase aperture: The shutter opens wider, allowing more light into the camera.
  • Decrease shutter speed: The shutter opens for longer, allowing more light into the camera.

Each option may reduce image quality in certain situations. Increasing ISO causes a general loss of quality, causing grainy images. Choosing a high ISO camera setting should be the last resort if a sufficient exposure can’t be achieved by increasing aperture or reducing shutter speed. These settings are often made by automatic exposure

Increasing Aperture Instead of ISO

When taking photos in low-light, one option is to increase the aperture. This will also reduce the depth of field, meaning only a very specific distance from the camera will be in focus; anything closer or further than the focal length will appear blurred. This soft-focus may be desirable, causing the subject to stand out from the background. In the below example, using a large aperture resulted in soft-focus of the background while allowing a fast shutter speed to clearly capture the rapidly moving horse.

Running horse in sharp focus while soft focus on background - achieved using large aperture with fast shutter speed to eliminate motion blur while limiting depth of field
Julian Herbert/Getty Images

In the below example, the depth of field is very small and only a small part of the subject is in focus. The aperture would need to be reduced to bring the whole bowl into focus.

Extreme example of limited depth of field
James Leynse/Getty Images 

Aperture is measured as an f-stop, which is the ratio of the focal length to the diameter of the aperture. A small f-stop, therefore, means a large aperture. Apertures for a high-quality, wide-angle lens may range from F/1.4 to F/8. Apertures for telephoto lenses are higher.

Reducing Shutter Speed Instead of ISO

If both your camera and everything in the photo will remain completely stationary, then a very slow shutter speed will result in sharp high-quality image. If there's noticeable movement while the shutter is open, this will create a blurred image. 

Very slow shutter speeds of up to a second or more require a tripod and a stationary subject, such as a landscape. Rapidly moving subjects, such as sports, require fast shutter speeds of 1/500 of a second or even less. Typical hand-held photos and portraits require shutter speeds of at least the focal length. For example, with an effective 50mm lens, you should use a shutter speed of at least 1/50 or a second.

Image taken at slow shutter speed shows motion blur on moving water while stationary plants are sharp
Craig Tuttle/Getty Images

The above image was taken using a tripod and a slow shutter speed. The stationary plants are in sharp focus while the moving waterfall shows a lot of motion blur.

When to Increase ISO

Ideally, you should always use a low ISO somewhere in the 100 to 400 range. This will produce finely grained photos that maintain detail when zoomed in to the individual pixels. If there isn’t enough light, first try to gain exposure by increasing aperture or reducing shutter speed. If it’s not possible to achieve enough exposure while keeping the subject in focus and avoiding unwanted motion blur, you'll need to increase the ISO.

With a conventional film camera, changing the ISO means physically loading a new film into the camera. An ISO of 400 would already be considered fast, and film speeds are typically limited to a maximum of 1,600. 

With modern digital cameras, changing the ISO is a simple setting. An ISO of 400 is considered quite normal and the sensitivity can be set much higher than with traditional film.

The maximum ISO settings are always increasing, but an ISO of over 100,000 may be possible. A high ISO camera will let you take good quality images in low light conditions, but remember choosing the highest ISO values will always result in a loss of image quality.