ISO
ISO controls how much a camera amplifies the signal its sensor already captured. A higher ISO (800, 1600, 3200...) brightens the resulting image without changing how much actual light reached the sensor, and doubling the ISO number is exactly one stop, matching the same doubling logic aperture and shutter speed use.
Because ISO amplifies rather than gathers light, raising it also amplifies the sensor's inherent noise floor along with the signal — a higher ISO generally means more visible noise, an unavoidable tradeoff for the extra brightness, distinct from aperture and shutter speed, which remain the only two variables that actually change the total light physically reaching the sensor itself.
Frequently Asked Questions
Is a lower ISO always the better choice for image quality?
For a correctly exposed image, yes — the lowest ISO that still allows your chosen aperture and shutter speed to expose correctly generally produces the cleanest result, since higher ISO always trades some noise for brightness.
What does ISO-invariant mean?
It describes how closely a sensor's noise matches whether you raise ISO in-camera or brighten an equivalently underexposed file afterward in post-processing — on a genuinely ISO-invariant sensor, the two different approaches produce very similar final visible noise levels.