Manual ISO is the ability to manually determine the ISO level of a camera – a setting that impacts how bright or dark a captured image is. One of the three pillars of photography (alongside aperture and shutter speed), ISO cannot be equated to sensor sensitivity in precise technical terms. Still, this common association helps understand the setting's function.
A higher ISO setting results in brighter images, whereas a lower ISO setting generates darker images. Nevertheless, ISO is only one of the factors to be considered when evaluating manual camera settings regarding photography goals and conditions of the photographed environment.
As with most things (especially when it comes to photography), an ISO choice is a tradeoff: higher ISO levels help produce brighter images, but they’re also linked to grainy visuals. For this reason, choosing a different shutter speed and aperture size may be options to consider before going for a higher ISO level. There are, however, situations in which adjusting ISO may be more desirable than adjusting the shutter speed – as an example, when documenting fast action and other cases in which motion blur is undesirable.
A growing number of consumers expect to be in charge of their camera settings, and a manual ISO feature is an integral part of being in control. This function used to be restricted to advanced camera systems; in recent years, however, it became a standard Android and iOS feature for high-end smartphones.