From: boyd@apt0.sao.arizona.edu   
      
   Optical wrote:   
   > Louis Boyd wrote:   
   >   
   >   
   >>>Do many people use the term "magnification" incorrectly when   
   >>>talking about reading glasses?   
   >>   
   >>Part of life is learning the vernacular use of words and phrases.   
   >>What's important, making  yourself understood or using a scientific   
   >>definition?   
   >   
   >   
   > My basic question was - do reading glasses perform any sort of   
   > magnification (when worn on your nose such that the lenses are located a   
   > fraction of an inch in front of your eyes) ?   
   >   
   > The fact that reading glasses may be labelled as or in terms of   
   > magnifiers is a side-tangent issue to my basic question.   
   >   
   > I was trying to make the point that when you quickly move a pair of   
   > reading glasses quickly on and off your nose, that they don't seem to   
   > make any apparent change to the size of text that's held close to your   
   > face, but they do change the focus of the text so that it's easier to   
   > read.   
   >   
   > So am I right - that referring to reading glasses as "magnifiers" is   
   > technically not correct?   
      
   Reading glasses only allow you to focus on an object when you move the   
   object  closer to you're face or your face closer to the object.   
   "Magnification" can be either a physical increase in image size (as with   
   a slide projector) or an apparent increase in image size (as with a   
   telescope.  Both are precisely measurable.  Moving an object nearer is   
   also an apparent magnification with the magnification being the ratio of   
   the two distance to the lens, usually the lens of an eye or a camera.   
      
   You are correct that just inserting them in your line of sight at the   
   position of your eye (as with contact lenses)  does not produce   
   significant  magnification.   I wear glasses with a +4 diopter   
   correction (250mm focal length).  When looking at distant objects they   
   produce  practically no apparent magnification.  Without them everything   
   is simply out of focus even at infinity.   They do give weak   
   magnification of nearby objects because the lens is not in the same   
   plane as the lens of they eye.  The purpose  of reading glasses however   
   is only to bring nearby objects into focus.   The "magnification" comes   
     primarily from reducing distance at which objects come into focus.   
      
      
   More properly glasses lenses are rated in diopters, not "magnification"   
   though the terms are often used interchangeably in common use.  The   
   diopter value of a simple lens  is the reciprocal of it's  focal length   
   in meters.  For "normal" glasses the number will be in the range of   
   about +6 to -6 though larger numbers are possible.  Prescriptions are   
   normally written in 1/4 diopter steps.   (you won't find a 3.3 diopter   
   lens commercially it will be 3-1/4  or 3-1/2).   
      
   In common English language usage glasses with positive diopter ratings   
   are still called magnifying lenses.  Your crusade against the common   
   language usage makes no sense to me.   What you may think are   
   "magnification" numbers are likely to be diopters which are a correct   
   lens specification.  The manufacturers marketing can call them anything   
   they want.   It may be labeled as  "magnification" instead of "diopters"   
   on the packaging as most non-technical US citizens and aliens don't have   
   a clue what the word diopter means.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)