The ISO setting is the third fundamental in determining exposure, alongside shutter speed (how long the sensor is exposed to the light) and aperture (how big the hole is between the lens and the sensor). It determines the sensitivity of the camera sensor to light.
The range is huge, just like shutter speed and aperture. Even entry level DSLR’s go from 100 (low sensitivity) to 6400 (high sensitivity) with typical options of 100 – 200 – 400 – 800 – 1600 – 3200 – 6400 – each setting is twice as sensitive as the last.
Each time ISO doubles, something else (shutter speed, aperture, or a bit of both) has to halve to maintain ‘correct’ exposure.
Generally speaking the lower the ISO (100 for example) the higher quality image, but don’t get too hung up on that; modern digital camera’s have an ever diminishing quality difference between low and high, and a high ISO can be used to creative effect (an old black and white print may be described as ‘grainy’ – in digital speak we would say ‘noisy’ – and may be exactly what you want)
Having three things to balance is what makes photography challenging – but is also what gives us the flexibility we need for different applications. There is never one single ‘correct’ combination of the three fundamentals.
Here’s three examples from both ends of the ISO scale and one in the middle:
- Low – Daylight landscape photography – I (almost) always use ISO 100 because the camera is always mounted on a tripod. In this situation a slower shutter speed is fine, as there is no risk of the camera moving during the exposure, and I benefit from the slightly higher quality images in return.
- Medium – Sports photography – the subject is always moving so the benefits of a faster shutter speed would far outweigh the tiny reduction in image quality that a higher / faster ISO (400, 800 for example) would create
- High – Astro photography – ISO 1600, 3200 or 6400 – here we are looking for maximum sensitivity to pick out those distant specks of light but have to accept some ‘noise’ may be generated. You may be interested in my article on photographing the milky way which uses a high setting.
In summary – ISO is one of three sides of the exposure triangle to get right – and if you lengthen the ISO side of the triangle then one, or both, of the others need to be shorter.