What is simple transposition of lens ?
The conversion of the prescription from plus cylinder form to minus cylinder form or vice versa, and changing the axis by 90 degrees is called transposition.
Type of transposition
Transposition is ophthalmic method to calculate the lens power or to change the form of lens power.
Basically transposition have two type :
2. Toric lens transposition
Rules of transposition in optics
There are three rules of transposition in optics add sphere into cylindrical power, change sign of cylindrical power, Change axis by 90 degrees.
Steps for transposition
• Algebraic addition of the spherical and cylindrical power including sign.• Change the sign of the cylindrical power.
• Either add or subtract 90° from existing degrees.
Uses of transposition of lenses
The most commonly use of transposition of lenses to change the form of lens power.
Change the spectacle power from plus form to negative form.
The most important benifit of transposition of lenses is most commonly lens power availability in minus form.
Some times we have get lenses in plus form for that time we have to do transpose the lens power negative form.
Lens transposition formula
Example of Simple transposition of lens:
(i) + 1.0 DS / + 5.0 DC @ 180°
Transpose:
Algebraic sum of the spherical and cylindrical power:
+ 1.0 DS + 5.0 DC = + 6.0 DS.
Change the sign of the cylindrical power: - 5.0 DC
Change the axis: 180-90 = 90°
Prescribe: + 6.0 DS /-5.0 DC @ 90°.
(ii) -0.5 DS / + 1.0 DC @ 17°
Transpose:
Algebraic sum of the spherical and cylindrical power:
-0.5 DS + 1.0 DC = + 0.5DS.
Change the sign of the cylindrical power: - 1.0 DC
Change the axis: 90 + 17 = 107°
Prescribe: + 0.5 DS / -1.0 DC @ 107°