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Basic Optical Principles
Prentice's Rule

Once it is found that either wanted or unwanted prism is present in any given lens and the direction of the base is determined, the power of the prism can then be calculated utilizing a simple formula called Prentice's Rule. It is important to become familiar with Prentice's Rule since it is used many times each day as a routine matter in the optical dispensary.

Prentice's Rule states: The power of the prism is equal to the power of the lens in diopters times the amount of decentration in millimeters divided by 10.

Stated Algebraically:

 = F x dec (mm) 10

where:

= prism diopters
F = power of the lens in diopters
dec = decentration or distance in mm away from the optical center of the lens.

Example 1: How much prism is induced 4 mm away from the optical center of a +3.00 D spherical lens?

Applying Prentice's Rule, F = +3.00 and dec. = 4.0 mm
so:

= F x dec/10
= 3.00 x 4/10
= 1.2

Example 2: A patient's PD was mistakenly noted to be 62 mm when the actual measurement is 66 mm. If the power of the lenses are ­2.00 diopters in each eye, how much prism will be induced in the finished spectacles and in which direction is the base?

 = (F x dec) / 10 Right Eye: (-2.00 x 2) / 10 = 0.4 Total = 0.8 Base out Left Eye: (-2.00 x 2) / 10 = 0.4

How do we know the direction of the prism is base out? The optical centers were positioned a total of 4 mm narrower than the actual PD. By referring to the figure D on page 13 which illustrates this situation with a minus lens. It can be observed that the direction of the base of the prism through which the patient is looking is oriented out, away from the nose.

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