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nominal power



steps for transposing from one form to the other

1. add the sphere and cylinder values to obtain the new sphere value
2.change the sign of the cylinder(plus to minus or minus to plus)
3. change the axis by 90 degrees (this can be by addition or subtraction since the end result is the same. The answer for the axis must be from 1 to 180 degrees. an answer of 190 is unacceptable.


O.D. stands for?

O.D. stands for oculus dextrus, meaning right eye in Latin.


• O.S. stands for?

• O.S. stands for oculus sinister, meaning left eye in Latin.



These are imaginary lines that help us describe the location of the axis powers of a lens with a correction for astigmatism.


spherical lens

In this type of lens, the power is the same in all directions.


cylindrical lens.

This type of lens looks like a vertical slice cut from the flattest surface of a grape.


sphere number

first number for each eye. This number always tells you if the eye is nearsighted or farsighted. We call this number
and it specifics a single plus or minus power in all directions.


plus lens

means that the eye is hyperopic or farsighted.


minus lens

This means that the left eye is nearsighted or myopic


second set of numbers?

These are called the cylinder numbers, and as you’ve seen, we use them if an eye has astigmatism.


final set of numbers for each eye.

This set of numbers describes the direction in which the person’s cornea is the flattest, causing the astigmatism. We call this number the axis or the axis of the astigmatism.


sph for sphere or D.S.

some people don’t have an astigmatism, so not all prescriptions will have this second part. If there’s no astigmatism, you’ll see the abbreviation sph for sphere or D.S. for diopters of sphere after the number,


The first set of numbers

refers to the right eye (O.D.)


the second set of numbers

refers to the left eye (O.S.).


cylinder numbers,

the second set of numbers we use them if an eye has astigmatism. The cylinder power tells us the difference between the steepest axis of the eye and the flattest axis of the eye, which are generally separated by 90 degrees. Typically in the optical field, we write the astigmatism correction as a minus cylinder number. (We can also write this number in plus cylinder form



When a prescription has an add, it indicates that the prescription is for a bifocal, trifocal, or multifocal lens.


Transposing a Prescription

Step 1: Take the sphere number (including the sign) and the cylinder number (including the sign), and add them together to get your new sphere.

Step 2: Take the cylinder number, and change its sign to the opposite sign to come up with the new cylinder.

Step 3: Take the axis, and change it 90 degrees by either adding 90 or subtracting 90 from it. Remember that the axis cannot be 0 or greater than 180. This will tell you whether to add or subtract.


Simple myopic astigmatism.

his is when there is no astigmatism in one meridian, and 90 degrees away from that meridian, the eye is nearsighted.


Simple hyperopic astigmatism

In this type, the eye is normal in one meridian. In the other meridian, 90 degrees away, it’s farsighted.


Compound myopic astigmatism.

Here, the eye is nearsighted in both meridians. However, it’s more nearsighted in one meridian than the other.


Compound hyperopic astigmatism

In this type of astigmatism, the eye is farsighted in both meridians, but it’s more farsighted in one meridian than the other.


Mixed astigmatism.

In this case, one meridian is nearsighted, and the other is farsighted.


optical crosses.

Using an optical cross will help you determine the power of a lens. To come up with the right numbers, you’ll add the front and back numbers for each meridian.


spherocylinder lens

If a lens has an astigmatism correction and a spherical correction for either myopia or hyperopia,


Single-vision lenses,

have the same power throughout the lens. That is, there’s no add like there is in bifocals or other multifocal lenses.


Spherocylinder lenses

are just lenses with a sphere (to correct nearsightedness or farsightedness) and a cylinder (to correct astigmatism).


Multifocal lenses

are lenses in which the lower portion of the lens has an add that makes it more magnified (or plus) in power than the rest of the lens.


segment, or seg for short.

The bottom part of the bifocal lens that contains the reading add


Seg depth or seg height:

the depth of a bifocal segment (that is, the distance from the top to the bottom of the segment).