Telescopes, light vergence, and accommodation.

This paper explains why the accommodative response to near objects is ineffective through a telescope. A simple formula is used to demonstrate the need for a bifocal spectacle objective or a change in vertex distance when viewing near objects with a contact lens spectacle lens telescope. Using this exact formula, clinicians can now more precisely compare refractions with and withoug a telescope. The formula also shows why refractions with telescopes used for driving should be accomplished at test distances of 20 feet or more.