When you make an ordinary hologram the wave propagation takes place in exact agree-ment with Maxwell's equations, even if you don't understand Maxwell. But if you simulate wave propagation in the computer you must understand exactly what you are doing since the computer cannot help you as nature does in an actual experiment. An unprogrammed computer is absolutely dumb, nature is not. Thus, producing computer holograms is an educational exercise. You gain intuitive insight into the fabulous sampling theorem. And you can also study systematically why it is sometimes good to use a diffuser in contact with the object. In other words, computer holography lets you explore by simulation. This subject will come up 'again at the end of this paper.