Introduction

Optometers are instruments designed to determine the accommodative or refractive state of the eye. In Chapter 10, we discussed a number of methods that could be used to measure the accuracy of focus of an optical system. However, in all those cases, the image plane was directly accessible. In the eye, this is not the case and therefore we have to access it indirectly. We can do this in two ways. In one method, we can ask the subject to make some judgement of the quality of the retinal image or focus level and we will call these subjective methods. Alternatively, a second person or observer examines the light reflected from the retina and makes a judgement of the focus error. We will call these objective methods. A number of optometers that are available make the focus judgements automatically, by electronic processing. These are based upon one of the objective methods and are beyond the scope of this book, but have been discussed by Bennett and Rabbetts (1989).

The ultimate accuracy of any optometer method is limited by the depth-of- field of the subject's eye. The depth-of-field of the eye is dependent upon the pupil diameter and typical values have been given in Chapter 13. Errors can also be caused by unwanted accommodation responses of the subject's eye. Many people accommodate when viewing through optical instruments (a phenomenon called instrument myopia) and we discuss this phenomenon further in Chapter 36. This has to be overcome when measuring refractive error and can be done by using fixation targets.