The Reduced, or Equivalent, Optical System
A complex multiple lens system can always be analyzed sequentially, but the process is tedious and must be repeated from the beginning for each source object location. However, we can calculate an equivalent optical system consisting of just 1 pair of principal planes, 1 pair of focal points, 1 pair of nodal points, and 1 power. The paraxial properties of this equivalent system, called the reduced optical system, are identical in every respect to the original system consisting of multiple lenses. Thus, once the equivalent system is determined, we need only a single vergence equation calculation to locate the image of any object. A simplified reduced model for the human eye (Figure 1-22) is discussed in Chapter 3.
Figure 1-22 Reduced optical system for the human eye. The retinal image size (apparent size) of an object depends in part on the angle subtended at the object nodal point N1. A small object close to the eye can subtend a larger angle than a large but distant object. This is the basis of the magnification produced by a telescope. The object in A is twice the size of the object in B. However, the object in B is 4 times closer to N1 than the object in A. Therefore, the angle the object in B subtends is twice as large as the angle the object in A subtends. C, The 2 angles are directly compared.
(Courtesy of Edmond H. Thall, MD.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.