Behold the Iris mechanism !
In order to control the amount of light entering through the lens, an iris diaphragm (see below) is incorporated into the camera.
Why should one be bothered about the amount of light entering ?
Well, too much light can wash out your image, and too little light can make
everything dark, so it’s important to have an iris that’s specifically
suited for the task to capture a crisp image.
The above simulation gives you a perspective on what happens inside: When you rotate the iris, the arm which is hinged to the rotating frame pushes in a blade that either closes or opens the aperture.