ISO affects the brightness produced from the sensor’s recorded signal. It does not add light or change how many photons arrived at a fixed aperture and shutter time. Raising ISO can help you use a necessary shutter speed in dim conditions.
Noise needs context
Images often look noisier when there was little light to record. Sensor behaviour, exposure, processing and how closely you inspect the picture all matter. Increasing ISO at fixed light, aperture and shutter time does not automatically make every camera’s noise worse.
Try it
In a half-lit room, fix the camera, aperture, shutter time and lighting. Turn Auto ISO off. Photograph the same scene at ISO 100, 800 and 3200. First compare brightness, then compare equally bright processed versions to judge noise fairly.
By Joseph Hubbard, photographer and artist at Hubbard.ART.
Continue Photography 101 and try the practical camera lessons · Browse more photography articles