From: rrllff@yahoo.com   
      
   Focal length is defined (for a thin lens) as the distance from the lens to the   
   image when the object is at infinity (a long way away).  The equation you have   
   been using is:   
      
   1/u + 1/v = 1/f   
      
   where u is the object distance from the lens, v is the image distance from the   
   lens and f is the lens focal length.   
      
   For a 1:1 magnification u and v will be the same.  From the above equation   
   this makes them each equal to 2f.   
      
   http://www.richardfisher.com   
      
   --- SoupGate-Win32 v1.05   
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