One of our lab assignments was to use a lens to focus an arrow-shaped beam of light onto a sheet of paper. We were then told to predict what would happen if we covered up the top/bottom/left/right half of the lens. I predicted that covering the top portion would cause the bottom part of the image to disappear, covering the bottom portion would cause the top part of the image to disappear, etc. As it turned out however, covering up any half of the lens only caused the image to appear about half as bright, but it was still completely whole. Can somebody help me understand what exactly is going on here? I have been attempting to draw ray diagrams to visualize it, but to no avail. Thanks.