From: invalid@nospam.com   
      
   On 12:56  30 Sep 2013, Helpful person wrote:   
      
   > On Monday, September 30, 2013 7:37:26 AM UTC-4, Elizabeth wrote:   
   >> Apologies if this is basic.   
   >>   
   >> I am trying to work out the diopter strength of two lenses. One has a   
   >> focal length of 18.5cm and the other 4cm.   
   >>   
   >> Is the diopter strength 5.4 and 25 respectively?   
   >>   
   >> Thank you for any help.   
   >   
   > That is correct.  The diopter power is the reciprocal of the focal   
   > length in meters-1   
   >   
   > However, if you are working with spectacles, the radius of the front   
   > surface (just this surface) is sometimes defined in diopters.  I   
   > cannot remember the definition but it is based on a specific   
   > refractive index and the expected power of the finished lens.  Don't   
   > worry about this unless you are designing/defining a spectacle lens.   
   >   
   > http://www.richardfisher.com   
      
   Thank you for the info.   
      
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