Introduction

The visual performance of a user viewing an image through an optical instrument depends upon a complex interaction between the eye and the instrument. Thus one cannot fully assess an optical instrument without a general understanding of visual optics and occasionally a knowledge of the visual capabilities of the individual user, which must include any anomalies of his or her visual system.

Many visual optical instruments are monocular and some people suffer visual discomfort when using monocular visual instruments for any length of time. Most of this probably stems from the tendency to keep one eye closed while viewing through the instrument. A binocular equivalent instrument will eliminate this problem and offers the possibility of a stereoscopic image. However, binocular instruments may lead to other problems. In this chapter, we will only discuss the ergonomics of monocular instruments and leave the ergonomics of binocular instruments and potential problems until the next chapter.

Instrument focussing

Image vergence

If an instrument is to be correctly focussed, the image must be within the accommodation range of the user. Classically, the ideal image vergence of visual optical instruments is taken as zero, that is the image is at infinity. The reasoning justifying this situation is that it was traditionally believed that the eye prefers viewing with relaxed accommodation. Unfortunately this approach neglects instrument accommodation and any refractive error of the user. The nature of instrument accommodation will be discussed later in Section 36.1.2. The different types of refractive errors and their distribution among the adult population has already been discussed in Chapter 13 and some representative values are shown in Figure 13.9.