To make a photograph with a camera the "shutter release" button is pressed. Pressing the button opens the "shutter". The shutter is like a door. It covers the hole in the camera box. The shutter is behind the lens. When the shutter is closed no light can enter the camera box. When the shutter is open light can enter the camera. When the button is pressed the shutter opens and then closes. This happens very fast. The amount of time the shutter stays open for is called the "shutter speed". The shutter speed can change between 1/1000th of a second (0.001 s) to a few seconds. Normally the time taken for the shutter to open and close is far less than 1 second. 