Archived posting to the Leica Users Group, 2001/08/14

[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]

Subject: Re: [Leica] M6 correction lenses--Win<>Win
From: "Mxsmanic" <mxsmanic@hotmail.com>
Date: Tue, 14 Aug 2001 12:23:18 +0200
References: <006901c124a3$2f405920$0201a8c0@Workgroup>

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).

Replies: Reply from "Julian Koplen" <jkoplen@mindspring.com> (Re: [Leica] M6 correction lenses--Win<>Win)
In reply to: Message from "Frank Filippone" <red735i@earthlink.net> ([Leica] M6 correction lenses--Win<>Win)