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This month: ISO All you need to know about the most important three letters in digital imaging
Of the three exposure musketeers, ISO is the most recent. That’s not because film speed hasn’t been around for a long time – because it has – but with film, it isn’t possible to change speed with the same joyous alacrity that we can on our digital cameras. Essentially, you are limited to the film’s quoted speed. Of course, you can always underexpose for richer colours or push-process, or shoot half the roll at one speed and the other at a different speed and then clip test during processing, but ultimately, your options are limited.
Furthermore, the fastest film you can buy is ISO 3200. That’s nothing when most digital cameras have a native range that tops out at ISO 12,800 or 25,600 – and many go way beyond if you include expanded speeds. The Nikon D6 and Sony A1 cameras, for example, both have native top speeds of ISO 102,400.
With digital ISO, you have an incredible amount of freedom. You can shoot one frame at ISO 400, the next at ISO 25,600 and then finish at ISO 100. This is why digital ISO is now a fully paid-up member of the exposure trinity and, in partnership with aperture and shutter speed, we now have a formidable set of tools to play around with. The aperture and the camera shutter physically determine how much light reaches the sensor and the actual settings you use are your responsibility, one you can wholly or partially delegate to the camera. How you use ISO on your digital camera is also up to you. It has an Auto ISO so you can leave it all to the camera, perhaps setting an upper limit to suit your quality threshold, or you can take total control and adjust ISO to suit the shooting scenario.
Read on as we take a deep dive into ISO, what it is and how you can use it to empower your photography…
Getting to grips with ISO
With film, ISO is a measure of its light-gathering ability, according to guidelines set by the International Organisation for Standardization. Film emulsion is formulated to deliver a specific ISO and should be exposed and processed for that speed.
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Digital ISO is more complex and there’s a great deal of misinformation around. For a start, ISO is not an acronym and an image signal is not simply amplified to give faster ISO speeds.
Essentially, all photographers need to know is that digital ISO, as specified in ISO 12232:2019, gives us a range of values that we can relate to film ISO and that a lower speed is preferred when there is more light a
