1. What is ISO

Originally ISO 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 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. Turn off long-exposure noise reduction when shooting in raw

When shooting in raw it pays to turn off long-exposure noise reduction because the processing time can be as long as the exposure itself. So, a two-minute exposure would require roughly two minutes of processing. Noise reduction in this situation is best applied to images in post-production – for more control and to save time when shooting.

5. Effect of high ISO on sharpness

As ISO is increased you will experience more grain in images. This grain will effectively break up sharp subject edges and reduce fine detail, which has the effect of softening images. There will always be times when you have to shoot at extremely high settings, but the trade-off here is achieving the desired combination of shutter speed and aperture at the expense of overall image quality.

6. Reciprocity failure no longer applies

Reciprocity is the idea that all exposures are mathematically equal, so if you increase ISO, for example, by two stops, you need to decrease the shutter speed or aperture by two stops or by one stop each to maintain the same exposure. With film, in general, reciprocity only works with exposures between 1sec and 1/1000sec owing to the limitations in film emulsions. But with digital, reciprocity works at all shutter speeds without detail problems or colour shifts.