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     A Distance from front edge of lens to front focal point 
     BF Back focal length 
     C.A. Clear aperture 
     CT Center thickness 
     D Distance from back edge of lens to back focal point 
     DIA Diameter 
     EFL Equivalent focal length 
     ET Edge thickness 
     f/# f-number (focal length ÷ entrance pupil) 
     FF Front focal length 
     H Height 
     H1 First principal point 
     H2 Second principal point 
     LLength 
     nC Index of refraction for red hydrogen C line (0.6563 microns) 
     nd Index of refraction for yellow helium line (0.5876 microns) 
     nF Index of refraction for blue hydrogen F line (0.4861 microns) 
     R1 First radius (moving from left to right) 
     R2 Second radius (moving from left to right) 
     R3 Third radius (moving from left to right) 
     R4 Fourth radius (moving from left to right) 
     S Object distance 
     S' Image distance 
     V1 First vertex 
     V2 Second vertex 
     W Width 
     D Abbe constant