Low vision aids and the presbyope.

When light rays emerge divergent from a low vision device, a presbyopic patient requires a near addition to focus the image. The magnitude of the divergence and the near addition power used by a presbyope can substantially alter the magnification effects of a device. This study examines the effects of different emerging divergences and near addition powers on the magnification of low vision devices. The emerging divergence of 54 fixed focus stand magnifiers was measured using trial lenses and a telescope. The magnifiers represented a range of powers and manufacturers. The emerging divergence was not predictable. It could not be correlated to magnifier power. Although mathematical formulas exist to calculate the effect of the divergence and near addition on magnification, tables were developed to simplify this determination.