Subject 13:Transposition Rule.

• Transposition rule• Target focusing for an astigmatic lens• Regular, 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’

• 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).

Focusing the target in an astigmatic lens

• e= -2D• C. axis= 120º

• c+e= +5D• axis= 30º

Regular, transposition and bicylindrical formulas

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: -2.50 sph - 1.00 cyl a 30º

sphere: {(- 2.50) + (-1.00)}= - 3.50 Dcylinder: {- (- 1.00)}= + 1.00 D.orientation: 30º + 90º = 120º

Bicylindrical formula: - 2.50 cyl a 120º - 3.50 cyl a 30º