We can make an afocal telescope into a surgical loupe with a working distance of one-third meter by adding a +3 D lens to the front of the telescope. The +3 D lens is called a collimating lens because its purpose is to redirect the rays of pencils of light diverging from an object one-third of a meter away so that they have zero vergence, before they enter the telescope. Because the telescope is afocal, the pencils of light will have zero vergence again when they exit the telescope, and a surgeon with emmetropic vision can effortlessly view the object and see it as larger, thanks to the angular magnification. The +3 D additional power can be added to the power of the objective lens instead of adding an additional lens in front of the afocal telescope (Fig 7-5).

Figure 7-5 The surgical loupe. The working add first collimates the light from the tip of the object, O. This bundle of collimated light enters the Galilean telescope at a particular angle, θ, to the optical axis. The same bundle emerges from the telescope still collimated but at an increased angle, θ′, where the magnifying power of the telescope is equal to the ratio of θ′ to θ. The tip of the object is seen by the eye as if it were at infinity, with the entire object subtending the angle θ′.

(Redrawn from Basic and Clinical Science Course Section 2: Optics, Refraction, and Contact Lenses. San Francisco: American Academy of Ophthalmology; 1986–1987:74. Fig 45.)

General Principles of Optical Engineering

More-elaborate designs of telescopes and microscopes use additional lenses, perhaps aspheric, to adjust the distance to the object (working distance) and combat the blurring caused by monochromatic and chromatic aberrations and diffraction. Apertures and antireflective lens coatings serve to eliminate undesirable rays.

Terminology

This section introduces some terminology you are likely to encounter elsewhere.

The aperture stop of an optical system is the opening—which could be the rim of one of its lenses or an empty diaphragm—whose edges limit the angular breadth of the pencils of light that are allowed to pass through the system to form the image of an object. For instance, the pupil of the eye serves as