Introduction of Contact Lens Power Calculation:

- Generally, we wear spectacle 12-14 mm(vertex Distance) away from our eye.
- But there is no significant gap between the cornea and contact lens, due to which contact lens power is different from spectacle power.
- Spectacle power may be only Spherical, Only Cylindrical and Sphero-Cylindrical.
- The formula to identify Contact lens power is:
CL = SP / 1 – (SP X VD)
- Here,
- CL = Contact Lens Power.
- SP = Spectacle Power.
- VD = Vertex Distance in Meter.
Contact Lens Power = Spectacle Power / 1 – (Spectacle Power X Vertex Distance).
How to convert Spherical Power into Contact Lens Power:

Example: Suppose Spectacle power in both eye is -5.0D and Vertex Distance is 12 mm. What will be Estimated Contact Lens Power?
- Here,
- SP = -5.0D
- VD = 12 mm or 0.012 m (12 / 1000)
- According to Formula:
- CL = SP / 1 – (SP X VD)
- CL = -5.0 / 1 – (-5 X 0.012)
- CL = -5.0 / 1 – (-0.06)
- CL = -5.0 / 1 + 0.06
- CL = -5.0 / 1.06
- CL = -4.7169
- The nearby available power is: – 4.75DS
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How to convert Sphero-Cylinder Power into Contact Lens Power:

- Conversion of Sphero-Cylinder Power into contact lens power is bit different. We can convert sphero-Cylinder power into contact lens power in 3 steps:
- Step-1: Make an optical cross & distribute the power meridian wise.
- Step-2: Convert the power of each meridian using formula: CL = SP / 1 – (SP X VD)
- Step-3: Now convert the optical cross into Sphero-cylindrical form.

Example: A Patient both eye power is – 5.0/- 3.0 x 180 and Vertex Distance is 12 mm. What will be the contact lens power?
Step-1: Make an optical cross & distribute the power meridian wise:
- If we distribute power in an optical cross, then we have:
- In One meridian (90 deg): -8.0DS
- In Other Meridian (180 deg): -5.0DS
Step-2: Convert the power of each meridian using formula: CL = SP / 1 – (SP X VD).
- In One meridian (90 deg), we have:
- SP = -8.0DS.
- VD= 12 mm or 0.012m.
- So, Contact Lens power will be:
- CL = SP / 1 – (SP X VD)
- CL = -8.0 / 1 – (-8.0 X 0.012)
- CL = -8.0 / 1 – (-0.096)
- CL = -8.0 / 1 + 0.096
- CL = -8.0 / 1.096
- CL = -7.29927
- Nearest available power: -7.25DS
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- In Other meridian (180 deg), we have:
- SP = -5.0DS.
- VD= 12 mm or 0.012m.
- So, Contact Lens power will be:
- CL = SP / 1 – (SP X VD)
- CL = -5.0 / 1 – (-5.0 X 0.012)
- CL = -5.0 / 1 – (-0.6)
- CL = -5.0 / 1 + 0.06
- CL = -5.0 / 1.06
- CL = -4.7169
- Nearest available power: -4.75DS
Step-3: Now convert the optical cross into Sphero-cylindrical form.

- Now, if we convert the optical cross power into Sphero-cylindrical power then we got: -4.75/ – 2.50 x 180.
- So, our initial spectacle power was – 5.0/- 3.0 x 180. After converting into Contact Lens Power, we got: : -4.75/ – 2.50 x 180.
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1 Comment
how can I know what degree for my contacts lens right eye…-075-050×180…left eye -050