Optical properties of low vision telescopes.

Angular magnification, resolving power, astigmatism, field of view, eye relief, and light transmittance of 157 low power telescopes comprising 25 models from seven manufacturers were measured. Roof-prism Keplerian telescopes provided about one-half the resolution, 30% lower transmittance, and more objectionable image flare than Galilean designs. The roof-prism was responsible for producing overlapping doubled images that appeared astigmatic-like. The prism further compromised rotational symmetry by deviating the image in a direction along the roof edge. The Keplerian telescopes, however, had about twice the field of view of the Galilean telescopes. Notably, several Keplerian telescope models from different suppliers were found to be optically identical, although they varied 50-100% in price.