>>3588913Think about how light is moving through the aperture and how it is limiting light coming into the camera. Light travels in straight line. Look at the attached image. Notice in the illustration that the image flips. Light from the top of the scene passes though the aperture and then onto the lower portion of the rear wall of the camera and vice versa. What the aperture is doing is reducing the amount light from other parts of the scene. So the smaller the aperture, the less other light, the sharper image.
give this a gander, skip to the 1 minute mark,
https://www.khanacademy.org/partner-content/pixar/virtual-cameras/virtual-cameras-1/v/optics1-final 