Introduction

In this chapter, we will discuss how we select the proper design form or configuration for both refractive and reflective image-forming systems. We will also consider fold mirrors and prisms since they have a significant influence on the system design configuration.

The proper system design form or configuration of an optical system is generally the key to a successful design effort. The term configuration here means the basic form of the system which includes not only the number of elements, but also the relative optical power and distribution of the elements within the lens system. For example, an achromatic doublet of two cemented elements, as shown in Fig. 8.1a is clearly different in form from a Cooke triplet, which consists of three separated elements, as in Fig. 8.1b , with two outer positive crown elements and a negative flint element at the center. The Cooke triplet can be used over wider fields of view than a doublet due largely to a reasonable degree of symmetry fore and aft of the central element, which is at or near the aperture stop. The doublet and triplet are very different configurations.

Figure 8.1: Doublets and Triplets

What if we were to add a single positively powered element immediately following a cemented doublet, as in Fig. 8.1c? Would the lens configuration be called a triplet? It certainly would not be a Cooke triplet as the symmetry is not present. This is a very different configuration or design form from a Cooke triplet.