Archived posting to the Leica Users Group, 2001/08/14
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Frank writes: > My normal right eye long distance prescription > is 1 diopters. Is that MINUS or PLUS ? Diopter is the reciprocal of focal length; 2 diopters = 500 mm. A positive diopter means that the optical system in question is converging, i.e., it is focusing an image onto a plane (such as film or your retina). A negative diopter means that the optical system in question is diverging, i.e., you look into it and you see any image that _appears to be_ a distance away equal to the diopter. So -0.5 diopters means that the eyepiece forms a virtual image that appears to your eye to be two meters (1/0.5) away. +1.0 diopters would be a converging lens, which would help with farsightedness. I believe negative diopters (diverging lenses) are used for nearsightness. > Do you add or subtract -0.5 to that number > to get the correct lens for the camera to > come out right? Depends. I that if you want a lens that, combined with the M6 lens, will match your prescription, then you subtract -0.5 from your prescription to get the required parameters for the additional corrective lens (that is, if you need -2.0, you subtract -0.5 from this to get -1.5, and that would be the number to use for your corrective lens). If you want a replacement lens, you just use your eyeglass correction by itself (since you are removing the original lens).