What makes a prescription?
If the eye were a perfect sphere then all we would need to correct vision would be a sphere lens. Though there are many people who can be corrected with a sphere lens many patients require a spherocylindrical lens because they have astigmatism.
A spherocylindrical lens is made up of sphere power and cylinder power.
Optometrists and ophthalmologists think of spherocylindrical lenses differently. Ophthalmologists think of adding the sphere and the cylinder. Optometrist think of grinding the cylinder into the sphere at a difference of 90 degrees. They are optically the same.
Because of this difference we have to learn to transpose spherocylindrical lenses.
- Add the cylinder to the sphere
2.Change the sign of the cylinder
- If the axis is 90 or greater subtract 90. If not add 90.
Lets do an example. I labeled each part of the prescription for clarity.
+1.00(Sphere) +.50(Cylinder) @70(Axis)
1.00+.50 = 1.50
+.50 x -1 = -.50
70+90 = 160
+1.50 -.50 @160
Also notice that the cylinder is positive. When the cylinder is positive it is a plus cylinder. When it is negative it is a minus cylinder.
What is a spherical equivalent?
Formula: Spherical equivalent = sphere + 1⁄2 cylinder
A spherical equivalent is used when dispensing contact lenses. To find the spherical equivalent:
Divide the Cylinder by 2
Add the cylinder to the sphere
Lets do an example:
+2.00 +1.00 @ 60
1.00 / 2 = .50
2.00+.50 = 2.50
There will be problems asking you to transpose and maybe a problem on finding the spherical equivalent. It is important to practice so that you don’t miss these questions.
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