Camera ISO: how sensitivity affects noise and brightness

ISO (sensitivity) is a setting that determines how much the camera amplifies the sensor's signal. Low ISO (50-200) gives a clean photo in good light; high ISO (1600-6400 and above) gives a brighter photo in the dark, but with noise.

ISO is like microphone volume

In plain words

ISO is like the volume on a microphone. If the singer sings loudly (lots of light), you do not need to turn it up - the sound is clean. If the singer whispers (little light), you turn the volume to the max - you can hear them, but the hiss gets louder too. That hiss in a photo is noise.

How it works

The sensor collects light and turns it into a signal.
ISO amplifies that signal to the required brightness.
The electronics' noise is amplified along with the signal.
Shutter speed, aperture and ISO together determine exposure.
Night mode and pixel binning reduce the need for high ISO.

High ISO is brighter but noisier

Where you'll see it

In the camera's Pro mode, the ISO setting. In auto mode the phone chooses it. A photo's EXIF data records the ISO it was taken at.

Does it matter when buying

Understanding ISO helps in Pro mode: in good light keep ISO low for a clean picture; in the dark, a longer shutter speed with a tripod is better than maxing out ISO. For everyday photos, auto mode does a good job.

Myths

Myth: High ISO is always bad. Reality: Modern sensors and noise reduction give acceptable quality even at high ISO.
Myth: ISO makes the sensor physically more sensitive. Reality: The sensor stays the same; ISO just amplifies the signal.

FAQ

What ISO should I use in daylight?

The lowest - 50-100.

Where does noise in photos come from?

From amplifying a weak signal and from the sensor's electronics.

How do I see a photo's ISO?

In the photo details in your gallery (EXIF data).