Back to Basics: ISO Explained

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The term “ISO”, when used in photography, refers to the speed rating of film. “ISO” stands for “International Organization for Standardization”, and this organization is responsible for defining standards for many, many industries and disciplines. With respect to photography, the film speed ISO rating system was created to standardize the quality of film throughout the industry. This ensured that regardless of the brand of film purchased by the consumer, the outcome could be expected be virtually the same when using similarly rated film (debates over the merits of one particular brand over another aside). For example, ISO 200 Kodak film could be expected to perform similarly to ISO 200 Fuji film. Also, the film speed spectrum (ISO 100, 200, 400, 800, etc.) would perform in a consistent manner and allow the photographer to anticipate his or her specific requirements for the lighting conditions available.

The ISO rating indicates the relative amount of light necessary to properly expose the film. The higher the number, the “faster” the film, or the less light is required to properly expose the shot. So, on a bright and sunny day, ISO 100 is commonly used to properly expose the shot. For dimly-lit indoor shots, ISO 800 might be more appropriate.

Even though the industry has shifted primarily to digital photography, the ISO terminology and methodology still applies. The only difference is that instead of exposing physical film, the camera’s sensors are being exposed to record the image. And, instead of having a roll of film dedicated to a single ISO rating, which has to be entirely used up before switching rolls, the photographer can shift between ISO ratings for each image taken. This comes in very handy for photographers who used to have to lug around multiple cameras, each loaded with film of a different ISO, to accommodate any photographic circumstance.

Some things to keep in mind about ISO ratings:

- The higher the ISO rating, the more “noise” will be present in the photograph. This is represented by a speckled, dusty, or grainy appearance, most notably seen in solid colors or dark backgrounds.
- The ISO rating, shutter speed, and aperture all work in tandem to properly expose the photograph (this is commonly referred to as “The Exposure Triangle”). When shooting in aperture priority mode, changing the ISO will also change the shutter speed (increasing the ISO increases the shutter speed, and vice-versa). When shooting in shutter priority mode, changing the ISO will also change the aperture (increasing the ISO increases the f-stop number – remember, the bigger the number the smaller the opening – and vice-versa). For a truly EXCELLENT demonstration of this concept, as well as how the shutter speed, aperture, and ISO all work together, visit this camera demo by Craig Hickman of Dry Reading.
- When going from a brightly lit space to a dimly lit space, or vice-versa, it is a common mistake to forget and leave the ISO on the wrong setting. You’ll notice this mistake when you review your pictures, but try to get into the habit of checking your ISO every time you turn the camera on.
- Remember, a low ISO number is not as sensitive to light as a high ISO number. So, for a bright environment use a lower ISO, and for a dim environment use a higher ISO.

Photo credit: “Smile” by Seyyed Mostafa Zamani on Flickr Creative Commons

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