Hologram illustrates diffractive surfaces, as well as the difference in image quality that comes from making the second principal surface of an optical system curved instead of flat. This is a hologram made from two point sources. One source is located at infinity, and the other is located at an on-axis point 30mm to the right of the hologram. After the hologram is exposed and developed, the construction sources are removed, and the hologram is used to form an image of objects located at infinite distance on the left on a surface 30mm to the right. The hologram used for this example is curved, with the substrate radius being 30mm, so the construction source lies at the center of the principal surface. Then rays coming from infinity through various parts of the hologram obey the sine condition, and the hologram is free from coma. If you make the hologram flat, you will have a huge amount of coma, as shown below.