What is ISO (Film Speed)?

ISO is one of the three camera components that control photographic exposure:

The Exposure Triangle. The balance of these elements create "normal" exposure.

The Exposure Triangle. The balance of these elements creates “normal” exposure.

ISO stands for International Standards Organization. It is a numerical value used by digital and film cameras alike to define the light sensitivity of the recording medium. This numerical value can range from 25 to several thousands (digital filmmaking continues to push the technology and make chips that are more and more sensitive).

ISO Film speed

Example of a correctly exposed image at ISO 160. Credit: Alex Ranaldi License: CC BY-SA 2.0

The higher the ISO number, the more sensitive to light the recording medium is. This means that less light is required to produce usable images. Conversely, the lower the ISO number, the less sensitive it is, which means more light is required to attain usable images.

A low ISO can range from 25 to 200. This ISO would be appropriate for shooting daytime exteriors since the sun is so bright! A typical high ISO would range from 800 to 3200. This would be better used for nighttime interiors or situations with little light.

In a nutshell:

High ISOMore SensitivityLess Light RequiredGood for nighttime interiors
Low ISOLess SensisitivityMore Light RequiredGood for daytime exteriors

The ISO number can also be referred to as ASA for American Standards Association, or EI for Exposure Index. For all practical purposes, these indexes are equivalent.

Additionally, ISO can also be referred to as Film Speed because it is defined by how quickly the image can be recorded with a given amount of light. Therefore, a high ISO number is called “Fast”, while a low ISO is referred to as “Slow”.

What if I use the wrong ISO film stock/camera setting?

Using the wrong ISO can produce unusable images (otherwise called “artistic” if that’s your goal). If for example, you have a film stock/camera setting that is too sensitive to light (a high ISO) during a daytime exterior (where sunlight is abundant), the image would be washed out/overexposed such as this one:

ISO photography film speed

Nighttime setting (a high ISO) used for a bright exterior.
Credit: Chris Fleming. License: CC BY-SA 2.0

And the opposite would be true. If you use a low ISO at nighttime, the result would be underexposure:

What if Film Speed?

Nighttime photography with an ISO that requires more light than available.
Credit: Ian T. McFarland License: CC BY-SA 2.0

Of course nowadays, with the advent of digital cameras, you should never make a mistake of this magnitude since the viewfinder will show you the exposure you’re getting with your current settings. Back in the day of film, cinematographers had to meticulously use the light meter to calculate exposure and ensure that the image to be recorded would be usable and not wasted!

It is also worth pointing out that the ISO numbers increase geometrically. This means that every time the ISO number doubles, the light requirement is cut in half. To illustrate, ISO 100 requires twice as much light as ISO 200 to produce the same exposure. This relationship between ISO number and amount of required light is important because it allows cinematographers to calculate how much light will be needed by switching ISO.

ISO and picture quality

Since we are able to control the camera aperture to block light, you are probably wondering why not always choose the ISO that requires less light (isn’t less requirement always better?). Well, as everything else in life, ISO has trade-offs. In a nutshell, an ISO that requires less light produces images with digital noise or grain:

A clean image from a low ISO vs . the noisy image from a high ISO. Credit: www.exposureguide.com

A clean image from a low ISO versus the noisy image from a high ISO.                                           Photo Credit: www.exposureguide.com

For the best image quality, you want to go with the lowest ISO possible and ensure that enough light is added to the scene, which is why studio lights are so powerful. This also explains why bright outdoor scenes usually have almost no grain/noise.

For the sake of providing examples, below is a list of ISO numbers and the light conditions where they may be useful. Of course, these combinations are for “normal” exposure, and they can be manipulated according to your creative goals:

  • ISO 50 (or lower): Bright sunlight
  • ISO 100: Bright overcast
  • ISO 200: Outdoors with some overcast or in shade
  • ISO 400: Very well-lit indoor or indoor during daytime
  • ISO 800: Outdoor during dusk
  • ISO 1600: Night interiors with some light
  • ISO 3200: Very dark night interiors

A Note on Film Cameras

In the golden days of filmmaking, the cameras were loaded with celluloid reels. Each reel had its own ISO rating. Thus changing the ISO was not as easy as flipping a switch. Digital cameras are far “easier” because they have CCD sensors whose ISO can be adjusted electronically.

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