ISO is the unit used to measure and adjust the camera sensor’s sensitivity to light. With film, the ISO setting (or film ‘speed’) is part of the film’s physical and chemical properties and can’t be changed. With digital cameras it’s possible to ‘turn up the volume’ on the sensor to make it more sensitivity to light.
Because you can now adjust the camera’s sensitivity from one shot to the next, the ISO setting has now become an everyday exposure adjustment and part of the ‘exposure triangle’, alongside the shutter speed and lens aperture.
ISO settings follow the same exposure doubling/halving principle as other exposure adjustments, making it easy to balance shutter speed, lens aperture and ISO adjustments in your head – though the camera will usually do this for you anyway.
This is the sequence of ISO settings :
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
ISO 3200
ISO 6400
ISO 12800
ISO 25600
Some cameras offer a wider range than this, occasionally with a lower minimum value and often with even higher ISO settings at the top end of the range.
It’s common for cameras to offer a ‘standard’ ISO range and an ‘expanded ISO’ range which goes even further. These ‘expanded ISO’ settings do not necessarily conform to the ISO standard of sensitivity and may not give acceptable image quality, but they do offer an equivalent ‘exposure index’ to these higher numbers, so your exposures will look correct at these expanded ISO settings.
Each whole step in ISO adjustments corresponds to a shift of 1EV (or ‘f-stop’ in the exposure, though cameras offer intermediate 1/3EV steps for ISO, just as they do for shutter speeds and lens apertures. This gives finer control than shifting exposures in full 1EV steps.
However, most photographers stick to using shutter speed and lens aperture adjustments alone to adjust exposure and keep ISO adjustments for situations where the light levels are low.
This is because increasing the camera’s ISO setting also increases the digital noise in the image, reducing the overall image quality. As a rule, photographers prefer to use the lowest ISO setting possible in order to get the best image quality.
The ISO setting is most closely tied to the shutter speed. In low light, you may need to increase the ISO setting in order to get a fast enough shutter speed to avoid camera shake. And in sports photography, it’s important to use a shutter speed that gives you the effect you want, whether it’s freezing fast-moving subjects or creative blur in panning shots. Both of these require shutter speeds that may not be possible without changing the camera’s ISO setting.
Many cameras offer an Auto ISO setting designed to take the tedium out of constantly adjusting the ISO in low light. The camera will automatically select the lowest possible ISO setting for each shot while making sure the shutter speed is high enough to prevent camera shake. This shutter speed will depend on the lens being used and its focal length, but the camera gets this information automatically from the lens.