The design of a zoom stand magnifier--a new low vision device.

A new stand magnifier has been designed for low vision patients and developed into a working instrument. This zoom magnifier has two ergonomic advantages over the currently available single lens magnifiers: it stays in focus (zooms) as the power is changed and it has a longer working space to allow both reading and writing with the same magnifier. This two-lens zoom magnifier contains a stationary plus lens and a movable minus lens. Using a symmetry condition, first order equations are presented and solved for the two lens powers. A family of three zoom magnifiers was designed--low, medium, and high power--each with a conveniently positioned sharp image with a 2-fold range of power and magnification. The equivalent viewing powers of the units (for a +2.50 D add) range from 6 to 23 D. The working spaces are approximately double that of comparable power single lens stand magnifiers.