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May 27, 2019

If you think of exposure like a triangle, shutter speed and aperture are two points on that triangle. ISO is the third – and least understood – point. Throughout history, ISO has been called by many names – ASA, GE, DIN and more – but in 1974, the International Organization for Standardization consolidated the ASA and DIN scales into the ISO scale. Like ASA and other designations before that, ISO is a way to measure the sensitivity of film. With digital cameras, it’s a way to measure the sensitivity of your camera’s sensor.

In the days of film, ISO measured the size of the silver halide grains in film’s emulsion. Film with higher ISO numbers had larger grains that made the films more sensitive to light. Photographers relied on high-ISO films for low-light situations and used low ISO films – or “fast” films – in bright situations.

In the digital world, ISO works much the same way. Instead of switching between more or less sensitive films, you can adjust the ISO setting to make your camera’s sensor more or less sensitive to light.

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