When a photo is taken, the shutter moves out of the way. This lets light come in through the aperture and make a picture on the film or electronic sensor. In many cameras, the size of the aperture can be changed to let in more light or less light. The amount of time that the shutter lets light through can be changed as well. This also lets in more light or less light. Most of the time, electronics inside the camera control these, but in some cameras the person taking the picture can change them as well.