16 - Microscopes

Summary

Introduction

The simple magnifier has an upper limit of magnification of about 20. Above this value, the lens becomes too small and the aberrations become too high to form a useful image. When higher magnifications are required, they must be achieved by a two stage process. Two stage magnification is possible by using two lenses as shown in Figure 16.1 and the extra complexity allows more freedom to control the aberrations. The first stage magnification is done by the objective and magnifications of between 10 and 100 are achieved depending upon the equivalent power of the objective. The objective forms a real, inverted and magnified image of the object. This image is further magnified by the eye lens. The eye lens is effectively a simple magnifier and therefore the upper limit of magnification is that of a simple magnifier, that is about 20. Therefore the upper limit of the magnification of the microscope as a whole is about 2000. Thus the extra magnification gained by a two component microscope over the simple magnifier is just that gained by the magnification due to the objective.

Construction and image formation

A microscope basically consists of two positive power lenses: the objective and the eye lens, as shown in Figure 16.1. The objective carries out the first stage of magnification and produces a real image of the object. The second lens (the eye lens) further magnifies the image. The objective is the aperture stop. Usually a field lens and field stop are used at or near the intermediate image plane in order to reduce vignetting and hence provide a wider field-of-view.