Effect of Magnification and Field of View on Reading Speed Using a CCTV

Abstract

Persons with untreatable visual disorders whose vision cannot be corrected by routine refraction require either optical or electronic aids to magnify the images of objects of interest. Spectacles, simple magnifiers, telescopes and Closed Circuit Television Systems (CCTVs) can all be used as low vision aids to assist persons with reading. CCTVs generally come in the form of a television camera connected to a video display monitor. Optical low vision reading aids are available in different powers, but as the eye-to-lens distance increases, the angular field of view and the magnification of simple plus lenses decreases [1]. Clinicians working in low vision have often observed that reading speed of low vision patients is reduced when the high magnification aids are used away from the spectacle plane, due to the limited number of words seen at one time. The usual clinical rule of thumb for prescribing optical aids is, therefore, to prescribe the lowest power which will enable the patient to read the required print size. It is hypothesized that this provides the widest possible field of view through the aid, when the magnifying system is placed as close as possible to the eye.