Prentice’s Rule

The rule states that as the prismatic affect of a lens increases the further away the view point is from the optical center. The stronger the lens power, the more prism is induced into the lenses. The formula is written as

△ = hcm x D where

△ = Prismatic Power

hcm = movement in cm (You will be given the movement in mm so do not forget to convert!)

D = dioptic power of the lens

Example)

For example if a patient is looking through a lens that 5 mm away from the optical center in a -10.00 lens then the prism they will experience will be

△ = 5 mm x -10.00 = -50.00 (this is in mm so we must convert).

50.00 / 10 = 5.00 △

The only thing missing here is the prism direction, it is important to note if the lens is plus power or minus power as that affects the direction. The direction and power of both lenses affects the total prism power experienced in both eyes. BU/BU, BD/BD, and BI/BO all have canceling effects. BU/BD, BI/BI, and BO/BO all have compounding effects.

Examples)

A lens that is 4.00 △ BU in the left and 4.00 △ BU in the right would experience how much prism in both eyes? Since the directions are both BU the powers would cancel (subtract). 4 - 4 = 0, so there is no prismatic power experienced by the wearer.

A lens that is 2.00 △ BI in the right and 1.00 △ BO in the left also has a cancelling affect. 2 - 1 = 1 △ or Prism experienced. The direction would be BI because the remaining (majority) of the power is in the RIGHT eye with BI direction. Your final answer would be 1.00 △ BI OD

Let’s say you have the same examples from above except the power is now compounding. 4.00 △ BU in the left and 4.00 △ BD in the right, you would now add these two powers. 4 + 4 = 8.00 △ of Dioptic Power OD.

A Lens that has 2 D BI in the right and 1 D BI on the left then the resulting prism will be compounding. 2 + 1 = 3 D Prism BI OD

The other note is to make sure that you are working in the correct meridian, we will keep it simple for the following examples in either the 90 or the 180. To convert the axis from 90 to 180 all you have to do is flat transposition. For example -1.00 -1.00 090 is working in the vertical plane and you need it to be on the horizontal. The transposed RX would be -2.00 +1.00 180

Practice Problems: Notate the Power and the Direction of the Prism

1) How much prism would a patient experience if her PD is off 3 mm in each eye nasally? The RX is;

-1.00 -1.00 @180

-1.00 -1.00 @090

2) How much prism would a patient experience if the OC was placed at the frame PD (64mm) and the patient PD is 34/30? The RX is;

-4.00 -2.00 180

-3.50 -1.50 090

3) How many cms is a lens displaced if the patient is experiencing 2 diopters of prism BI on the left eye? RX is:

-12.00

-10.00

4) How much prism will a patient experience if the glasses are made at a 70 PD but the patient’s PD is 64? RX is;

-5.00

-6.50

5) How much prism will a patient experience if the OC was placed at half of the B measurement of a frame when it should of been at 20 mm for the OC.

Box: A = 55 B = 32 DBL = 18 ED = 55

RX =

+7.00

+6.75

6) How much prism will a patient experience if their PD was switched from 34/32 to 32/34 with an RX of

-600 -2.00 058

-4.50 -3.00 120

7) A patient needs 2 Diopters of prism BU OD and 2 diopters of prism BD OS. How far do the lenses need to be moved to obtain this prism?

OD: -8.00

OS: +6.00

8) Your patient is just now pickup up his eyeglasses and he does a lot of reading, he has an RX of +12.00 in both eyes and looks below the OC of his lenses 7mm to read his ipad. How much prism will he experience in these frames?

Answers:

1) 0.90 BO OS

2) 0.2 BI OS

3) 0.17 cm or 1.7 mm

4) 3.5 BI OD & 3.5 BI OS

5) 0.1 BD OD

6) 0.14 BO OD

7) 1.25 mm down OD & 1.67 mm down OS

8) No prism since both lenses are cancelling (BD OU with 8.4