Introduction

As magnifying devices, simple magnifying lenses are restricted to short working distances. The magnification and working distance are closely related and therefore single lens magnifiers are only useful for magnifying objects close to the lens and the higher the magnification, the closer this distance has to be. However, if one goes to two lens systems, magnification and working distance can be made independent and in principle, one can design a two lens system to give any magnification for any working distance. The microscope may be regarded as an exception to this rule in the sense that the higher magnifications lead to shorter working distances. Thus microscopes act more like simple magnifiers and do not make full use of the potential independence of working distance and magnification that are possible with a system consisting of two lenses.

In this chapter, another special case will be considered, one in which the working distance is infinite or very long. The completely general case will be left until the next chapter. The class of instruments used for providing magnification for very distant objects are called telescopes. These are usually regarded as afocal; that is their equivalent (refractive) power is zero. Telescopes are not only used as magnifying devices. Their unique properties make them ideal as aligning or focussing devices in a range of optical instruments. Binoculars are made from two identical telescopes with parallel optical axes. Telescopes may be classified as either refracting or reflecting. The distinction is the type of leading component called the objective. In a refracting telescope, the objective is a lens or lens system. In the reflecting telescope, the objective is a mirror or mirror system.