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What the 'devil' is ISO? By Mark Blezard

Updated: Jan 28, 2021

Film speeds explained.

Two of the most important elements of taking great photographs are exposure and focus. Composition can wait, you can always adjust this in Photoshop, whereas exposure is difficult to correct and focus almost impossible.

Exposure is governed by three settings on your camera, aperture and shutter speed you probably already know about, but ISO? ISO, or ASA as it was more often referred to pre-digital, stands for International Organization for Standardization. The ISO indicates how sensitive the sensor or film is to light.

For darker scenarios, you’ll need to use higher or more sensitive ISO settings to capture light, such as 800 or 1600 ISO. For normal daylight 100 is more the norm.

Pre-digital, it was an indication of how thick the light-sensitive emulsion on your film was. Really fast film, ISO 1600+, would be very thin and therefore require less light to fully expose. However, as with digital cameras too, the higher up the ISO scale you go the poorer the quality of the image because there is less emulsion/digital data holding the image. Images become more grainy and lack contrast.

So, in summary, it is your third tool to manage/manipulate exposure but always consider the output requirement (size of the final image) because the lower you go (ISO number) the more 'image meat' you capture. In the old days this was measured as dpi (dots per inch ability when printing), today it is pixels.

ISO photography explained by Mark Blezard
The ASA (also known as DIN) dial on my old, trusted Canon A1

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