Field of view and equivalent viewing power of near-vision telescopes.

There are often substantial differences in linear field of view (FOV) and equivalent viewing power (EVP) when a distance telescope is either fitted with an objective lens cap or adjusted in length to focus for near vision. In addition, larger differences in linear FOV are found for Galilean than for Keplerian telescopes of similar magnification and EVP. Simple formulas are presented here that make use of EVP as a factor for calculating the linear FOV ratio between the two methods of converting distance Galilean and Keplerian telescopes for near vision. Examples are provided to illustrate the clinical application of these formulas. The information presented here may be helpful when making recommendations concerning the most appropriate method of converting a telescope for near vision.