ISO and image noise
In short
- ISO sets how much the camera amplifies the light signal.
- Doubling ISO (200 → 400) brightens the image by one stop.
- Higher ISO adds noise: grain and blotchy colour, mostly in shadows.
- Use the lowest ISO that still allows the aperture and shutter speed you need.
From film speed to sensor gain
The name comes from film. Film was sold by sensitivity, and the standard for measuring it was set by the International Organization for Standardization, ISO. Faster film (ISO 400, 800) needed less light but looked grainier.
A digital sensor has one fixed sensitivity. Raising ISO tells the camera to amplify the signal more, the way you turn up the volume on a quiet recording.
Where noise comes from
Light arrives as individual particles. In dim light, few of them reach each pixel, and the random differences between neighbouring pixels become visible. Amplifying a weak signal makes that unevenness easier to see. This is noise: speckles, grain and colour blotches, strongest in dark areas.
The comparison above shows the same scene at rising ISO settings. Detail softens and grain grows as ISO goes up.
How high can you go?
It depends on the camera. Larger sensors collect more light per pixel and stay cleaner at high ISO. As a rough guide for modern cameras: ISO 100 to 800 looks clean; 1600 to 6400 is usable with some noise; above that, noise becomes obvious.
A slightly noisy sharp photo is almost always better than a clean blurry one. If the choice is between raising ISO and getting camera shake, raise the ISO.
Practical tips
Expose correctly at a higher ISO rather than underexposing and brightening later: brightening a dark file reveals even more noise.
Auto ISO with a minimum shutter speed is a good everyday setting.
Noise reduction in editing software can help, but it also smooths fine detail.