Abstract

In clinical optics relating to near vision there are five different popular definitions of magnification and confusion abounds. Each definition depends on different assumptions about the before and after viewing conditions. An alternative (equivalent viewing distance or EVD) is being offered as a means of describing the relative effect of near vision magnifiers. The EVD is the observation distance at which the object subtends an angle that is equal to the angular size of the image. For clinical purposes, the EVD for a microscopic spectacle lens is equal to the object-to-lens distance; the EVD for a fixed focus stand magnifier is equal to the image-to-eye distance divided by the lateral magnification; the EVD for a video magnifier is the observation distance divided by the lateral magnification; the EVD for a near vision telescope is the object distance divided by the magnification of the base afocal telescope, and the EVD for hand-held magnifiers can be derived by simple calculations involving the lens power, the accommodation, and the eye-to-magnifier distance. There are simple clinical methods for determining the key optical parameters of these various kinds of low-vision aid.

© 1985 Optical Society of America