STUDIES

# Spectacle Skills

# 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.

# Transposing

To transpose:

- 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

Answer:

+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

Answer:

+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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