In this chapter we will use the concept of thin lenses to understand some special strategies and techniques used in designing lenses. While it is sometimes possible to rely solely on a lens design program to improve a lens, this strategy usually leads to unwieldy components that are impossible to fabricate. By using the insights derived from a study of simple lens arrangements, the lens designer can see which variations in lens parameters will yield the greatest improvements in system performance, and still give designs for lenses that can be made and mounted easily and economically.
In many situations it is extremely useful to consider lenses in terms of thin lenses, rather than as a sequence of single surfaces. When we do this, we assume that the lens is of infinitesimal thickness, so that a paraxial ray will intersect the two surfaces of the lens at the same height. Naturally, real lenses are never thin in this sense, but the results derived using this assumption are valid for many real lenses. We will see from many of the lenses discussed in this text that thin lens Seidel theory provides insights into the possibilities of a new lens type, with little or no calculation.
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