The ISO button controls how sensitive the sensor is to light.

A higher ISO is used when the light is dim.

On many cameras, you change the ISO by holding the button down, and turning a control knob.

Make it a habit to check the ISO setting when you turn your camera on.

Let's say, yesterday, you were photographing at dusk.

You had the ISO set to 1600.

Today, it's sunny.

The camera will still be at ISO 1600 unless you remember to changeto a sunny-day setting of ISO 200.

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