Camera Metering - How To Get The Right Exposure

When you take a photo, you press the shutter button. The camera takes a meter reading of the light before the shutter opens. The shutter then opens, exposing the camera sensor to light.

The amount of light that gets through to the camera lens is determined by the aperture selected and the amount of time that the shutter is open. The ISO determines how the camera processes the light that reaches the camera sensor.

Depending on which mode you are in, the camera selects the aperture, shutter and ISO to get the correct exposure having taken a meter reading. This is all done in the time between you pressing the shutter button and the photo being taken.