ISO
The camera brightness setting that boosts the captured signal; higher values help in low light but reveal more noise.
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What it is
ISO is the camera setting that decides how strongly the captured sensor signal is turned into a visible image. The scale is set by the ISO 12232 standard, which is where the name comes from. Double it from ISO 100 to 200 and the result is one stop brighter at the same aperture and shutter speed. Unlike film speed, digital ISO does not make the silicon physically more sensitive; it changes amplification and processing after the light has already arrived.
Raise it when the alternative is a shutter too slow to hold or an aperture too wide for the focus you need: concerts, indoor events, night streets, or any moving subject in weak light. Modern cameras can make very usable pictures at ISO 1600 or 3200, especially when a sharp noisy frame beats a clean blurred one.
Gotcha: high ISO reveals noise, but starving the sensor of light is the root problem. Shooting too dark at ISO 100 and pushing the file several stops later is not a clean escape. Add light, open the aperture, or slow the shutter when those choices are available, then use the ISO you need.
Ask AI for it
Shoot a low-light concert photograph at ISO 3200, f/2.8, and 1/250 second, freezing the singer mid-gesture under colored stage light. Keep fine luminance noise visible and film-like instead of smearing skin into plastic. Finish the RAW file in Adobe Lightroom Classic with Photo > Enhance > Denoise at Amount 40, retaining hair, fabric, and microphone texture.