What Image Quality Is All About

Image quality is never perfect! While it would be very nice if the image of a point object could be formed as a perfect point image, in reality we find that image quality is degraded by either geometrical aberrations and/or diffraction. Figure 3.1 illustrates the situation. The top part of the figure shows a hypothetical lens where you can see that all of the rays do not come to a common focus along the optical axis. Rather, the rays entering the lens at its outer periphery cross the optical axis progressively closer to the lens than those rays entering the lens closer to the optical axis. This is one of the most common and fundamental aberrations, and it is known as spherical aberration. Geometrical aberrations are due to the failure of the lens or optical system to form a perfect geometrical image. These aberrations are fully predictable to many decimal places using standard well-known ray trace equations.

Figure 3.1: Image Quality, Geometrical Aberrations (Top) and Diffraction Limited (Bottom)

If there were no geometrical aberrations of any kind, the image of a point source from infinity is called an Airy disk. The profile of the Airy disk looks like a small gaussian intensity function surrounded by low-intensity rings of energy, as shown in Fig. 3.1, exaggerated.

If we have a lens system in which the geometrical aberrations are significantly larger than the theoretical diffraction pattern or blur, then we will see an image...