Adjustable focus telescopes for near vision

It is important for clinicians to know the equivalent viewing power and corresponding equivalent viewing distance of the adjustable-focus telescopes they prescribe for low vision patients. Formulas are presented here to calculate the equivalent viewing power and distance of these telescopes. The optical effects of adjusting a telescope for near vision are discussed and compared to using a lens cap fitted on the objective to permit focusing on a near target. Commonly prescribed adjustable-focus telescopes of four representative magnifications were evaluated. The results are presented in the form of a nomogram which can be used for determining the equivalent viewing distance of these telescopes based on the front vertex focal distance of the focused device. Other considerations including sources of error and field of view are also addressed.