formulas

Question 1

nominal power

Answer 1

F1+F2=Ftotal

Question 2

steps for transposing from one form to the other

Answer 2

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.

Question 3

O.D. stands for?

Answer 3

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

Question 4

• O.S. stands for?

Answer 4

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

Question 5

meridians.

Answer 5

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

Question 6

spherical lens

Answer 6

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

Question 7

cylindrical lens.

Answer 7

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

Question 8

sphere number

Answer 8

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.

Question 9

plus lens

Answer 9

means that the eye is hyperopic or farsighted.

Question 10

minus lens

Answer 10

This means that the left eye is nearsighted or myopic

Question 11

second set of numbers?

Answer 11

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

Question 12

final set of numbers for each eye.

Answer 12

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.

Question 13

sph for sphere or D.S.

Answer 13

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,

Question 14

The first set of numbers

Answer 14

refers to the right eye (O.D.)

Question 15

the second set of numbers

Answer 15

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

Question 16

cylinder numbers,

Answer 16

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

Question 17

add

Answer 17

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

Question 18

Transposing a Prescription

Answer 18

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.

Question 19

Simple myopic astigmatism.

Answer 19

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

Question 20

Simple hyperopic astigmatism

Answer 20

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

Question 21

Compound myopic astigmatism.

Answer 21

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

Question 22

Compound hyperopic astigmatism

Answer 22

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

Question 23

Mixed astigmatism.

Answer 23

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

Question 24

optical crosses.

Answer 24

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.

Question 25

spherocylinder lens

Answer 25

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

Question 26

Single-vision lenses,

Answer 26

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

Question 27

Spherocylinder lenses

Answer 27

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

Question 28

Multifocal lenses

Answer 28

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.

Question 29

segment, or seg for short.

Answer 29

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

Question 30

Seg depth or seg height:

Answer 30

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

Question 31

Seg drop:

Answer 31

The distance from the top of the full lens to the top of the segment.

Question 32

Compensated power

Answer 32

is the new power that the new lens will need to have in order to have the same effective power as the original lens when it is placed in the new vertex distance.

Question 33

how to use an optical cross

Answer 33

1. First, you’ll clock the front of the lens in two directions. (This will give you the highest and lowest powers for the front of the lens.) Let’s say the front of the lens is spherical, so you get the same number—+800—in the horizontal (180-degree) and vertical (90-degree) meridians. But when you follow the same steps for the back of the lens, you come up with two different powers: -6.00 and -4.00.Let’s draw a cross to represent our meridians. Our first cross shows +800 in both the vertical and horizontal directions. (That’s the front of the lens.) Our second cross shows -6.00 in the vertical direction and -4.00 in the horizontal direction. (That’s the back of the lens.)

+8.00 -6.00

┼ +8.00 ┼ -4.00

Front Back

2.Your next step is to add the front and back powers for each meridian. In our example, you’ll add the 90-degree reading for the front of the lens to the 90-degree reading for the back of the lens. Next you’ll add the 180-degree reading for the front of the lens to the 180-degree reading at the back of the lens. Here’s what you’ll get:

+8.00 -6.00 +2.00

┼ +8.00 ┼ -4.00 ┼ +4.00

3.That final cross tells you your lens powers. Since the final power of your lens has two different powers, you know it’s a spherocylinder prescription.
As an optometrist, I use negative cylinder form. So I’d write the prescription of this lens as:

+4.00 – 2.00 x 180

In order to get a minus number in the cylinder portion, I would write the prescription starting with the +4.00 and then subtract 2 to get -2.00. Since the +4.00 power is in the 180 direction, the axis is 180.

Question 34

Prentice’s Rule:

Answer 34

Prism diopter (Δ) = Power of the lens (F) x the decentration distance in centimeters (dcm) between the optical center and the center of the pupil

Prism = F x d

Δ = lens power x decentration distance in centimeters