4.1 Introduction It was shown under Chapters 2 and 3 that the “thin lens” concept is an extremely useful one for preliminary calculations and analysis as well as a general design tool. Determining the difference in performance between that fictitious thin lens and a realistic thick lens is an interesting exercise which confirms the statement above. We begin our exercise with a thin lens shaped for minimum spherical aberration, add a reasonable thickness, and modify only the rear surface radius to maintain the focal length of the lens. We then calculate the spherical aberration for both cases and compare the results for three f∕2, 100-mm-focal-length lenses. The first lens is made from glass with an index of refraction of n=1.5 for the visible spectrum (VIS). The second is for the infrared mid-wave region (MWIR, 3–5 I¼m), made from silicon with n=3.4. The third singlet is made from germanium, n=4, for the application in the infrared long-wave region (LWIR, 8–12 I¼m). 4.2 Adding a Thickness and Changing the Second Surface Radius The focal length for a thin lens (one for which its thickness is assumed to be zero) is expressed by 1 f =(n−1)(1 R 1 −1 R 2 ).