What part of camera lens lets light through?

The iris (technically the diaphragm) lets light from the lens enter the camera.

The entire lens can let light fall on the film or sensor. But, much of the time it doesn't do that. An device called an iris makes the hole through which light passes larger and smaller. When all the way open it lets light from the entire lens reach the film or sensor, but it can make the hole smaller and only allow light from a part of the lens enter.