Archived posting to the Leica Users Group, 2001/08/14
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The original M eyepiece lens is NOT removed when the correction lens is installed. The correction lens is in a mount which screws in over the factory lens. Thus, to arrive at your final effective lens power, you add the values of the factory lens and your custom lens, taking negative numbers into account. Therefore, (factory lens) plus (custom lens)=(your eyeglass distance prescription). And you have to know if your eyeglass distance prescription is a plus or minus value. Therefore, if your distance correction is, say, -1.0 diopter, you would need a -0.5 custom lens. That, when added to the factory -0.5, would yield your -1.0. If your distance correction is, say, +1.0, then you would need a custom lens of +1.5, which, added to the factory -0.5, would yield your desired sum of +1.0. Part of the trick is in knowing what the factory has already put into its own eyepiece. Does this sound right? Do we have any eye doctors on the LUG? Julian - ----- Original Message ----- From: "Mxsmanic" <mxsmanic@hotmail.com> To: <leica-users@mejac.palo-alto.ca.us> Sent: Tuesday, August 14, 2001 6:23 AM Subject: Re: [Leica] M6 correction lenses--Win<>Win 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).