From: rrllff@yahoo.com   
      
   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   
      
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