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# Subject 13: Transposition Rule.

Subject 13: Transposition Rule. Transposition rule Target focusing for an astigmatic lens Regula r , transposition and bicylindrical. Transposition Rule.  º c e T  ’º c’ e’.  ’ =  + 90º c’ = - c e’ = e + c. Transposition Rule.  º c e T  ’º c’ e’.

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## Subject 13: Transposition Rule.

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1. Subject 13:Transposition Rule. • Transposition rule • Target focusing for an astigmatic lens • Regular,transposition and bicylindrical

2. Transposition Rule º c e T’º c’ e’ ’ =  + 90º c’ = - c e’ = e + c

3. Transposition Rule º c eT’º c’ e’ • The two f.e.c. Correspond to the same lens, with the same marking. • The lens has not been modified in any way, only a change in the analytical process has been made. • The cylindrical components of both formulas always have opposing signs (positive and negative).

4. Focusing the target in an astigmatic lens • e= -2D • C. axis= 120º c+e= +5D axis= 30º

5. Transposition formula: - 3.50 sph +1.00 cyl a 120º sphere: {(- 3.50) + (+1.00)}= - 2.50 D. cylinder: { - (+ 1.00)}= - 1.00 D. orientation: 120º - 90º = 30º Regular, transposition and bicylindrical formulas Regular: -2.50 sph - 1.00 cyl a 30º sphere: {(- 2.50) + (-1.00)}= - 3.50 D cylinder: {- (- 1.00)}= + 1.00 D. orientation: 30º + 90º = 120º Bicylindrical formula: - 2.50 cyl a 120º - 3.50 cyl a 30º

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