If you’re new to photography you may have asked yourself, ‘What is ISO?’
Back in the days before digital, film came in a variety of different speeds. The ‘faster’ the film, the more sensitive it was to light – allowing you to use faster shutter speeds than with ‘slower’ film.
Using these higher-sensitivity film emulsions was useful for moving subjects – and particularly so in low light. This film speed was measured using a number of different scales – with two of the best known, the American ASA and German DIN scales, eventually being brought together to give us the standardised ISO system.
With film cameras, exposure is controlled by aperture and shutter speed settings at a fixed ISO setting (which is dictated by your film). Digital cameras allow you to change the ISO setting with just the push of a button, but on most models this can be performed automatically with your ‘Auto ISO’ function.
This is very useful when the lighting conditions are changing quickly – when you’re moving from outdoors to indoors, or from bright sunlight into shadows.
Shooting indoor sports action using a fast shutter speed, old, gloomy churches without flash or bright landscapes where you want to slow down the shutter speed to capture movement are just some of the many situations where you will need an extreme ISO to get better results.
ISO denotes how sensitive an image sensor is. Any change from the manufacturer’s native ISO (the lowest default, which produces the optimum image quality) will have some form of electrical signal modification that results in noise.
Most DSLRs have a native ISO of around 100 or 200; beyond that, at the extreme low end of the range, quality isn’t improved. However, some manufacturers offer lower ISO values in the menu, such as ISO50. Others use a decimalised f-stop value to indicate when it falls below the native ISO. Both are great for using wide apertures or long shutter speeds in bright lighting conditions.