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Lens Transposition

lens tranposition for making glasses
It is a technical formula to change the lens form from one type to another. Simply transposition are used to convert one form to another, usually "+" to "-".

Some practitioner prescribe eyeglasses in what is called plus cylinder, and others do so in minus cylinder. Regardless of which is used, the prescriptions are the same.This can be proven by transposing the eye wear prescription. 

There are two types of lens transposition.
  1. Simple transposition
  2. Toric Transposition 
Now let's discuss about the simple transposition. 

Follow the three rules of transposition.
  1. Sum : Add the sphere and cylinder powers of prescription to determine the new sphere power.
  2. Sign : Change the sign of Cylinder
  3. Axis : Change the axis by 90 degree (if axis is less then or equals to 90°, add 90° to the axis, and if axis is more then 90°, subtract 90° from the axis). 
For Example:
  • +3.50D Sph / +2.00 Dcyl × 140°
Answer:
  • New Sphere power  = + 5.50 D sph (Add spherical +3.50 and cylindrical power +2.00)
  • New Cylinder power =  2.00 D cyl (Sign Changed)
  • New Axis = 50° (140°-90°)
  • Final Transposed Power is: +5.50D Sph / -2.00 Dcyl×50°
Another Example:
  • +3.00 D Sph / -1.50 Dcyl  x30 °
Answer
  • New Sphere power  = + 1.50 sph (Add spherical +3.00 and cylindrical power -1.50)
  • New Cylinder power = + 1.50 D cyl (Sign Changed)
  • New Axis =  120° (30°+90°)
  • Final Transposed Power is: +1.50D Sph / +1.50 Dcyl × 120°
watch this video below to know more about lens transposition :




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