In the know: High ISO

High ISO settings are often needed in low light and for faster shutter speeds. James Abbott shows how you can embrace it while maintaining image quality and sharpness

- James Abbot

1 What is ISO?

Originally, ISO was related to film speed; it refers to the sensitivity of the emulsion to light. Digital sensors work in a similar but different way. With digital sensors, when shooting at lower ISO settings the sensor appears to be less sensitive to light, and image quality is better, while at higher settings noise becomes more apparent as the sensor becomes more sensitive to light. The reality, however, is that digital cameras have a single base ISO setting, which is often ISO 100, and when increased or decreased, a software algorithm simulates an effective sensitivity of the sensor.

2 Test your camera's ISO performance

The best ways to identify the highest ISO setting you're happy to use is to do an ISO test. Set your camera up on a tripod in aperture-priority mode and take a series of shots at all the native ISO settings. Shoot in raw and load the images into Lightroom or your preferred raw-editing software and make sure all noise reduction is switched off. From this, you can see exactly where image quality becomes unusable.

3 The exposure triangle

Working with the simple idea that digital ISO levels are the same as film ISO levels, ISO is part of the exposure triangle that also includes aperture and shutter speed, the combination of which dictates whether an image is correctly under or overexposed.

A stop with shutter speed and ISO is easy to understand; you simply double or halve the number. So you might go from ISO 100 to 200 to 400 and so on, while with shutter speed you could go from 1/125sec to 1/60sec to 1/30sec, etc. Aperture is however different, and the most common are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.

4 Effect of high ISO on sharpness