Calculate differential prismatic effect with Prentice’s rule from each lens power and the reading drop, or find the difference between two prism values.

Uses Prentice’s rule: prism (ฮ”) = distance from the optical centre (cm) ร— lens power (D).

For a sphero-cylinder, use the power in the meridian you look along (vertical for reading).

For a sphero-cylinder, use the power in the meridian you look along (vertical for reading).

Typical reading position is about 8โ€“10 mm below the optical centres; 10 mm is the usual textbook value.

Viewing position

Differential Prismatic Effect Formula

The following formula is used to calculate the differential prismatic effect between two lenses.

DPE = P1 - P2

Variables:

  • DPE is the differential prismatic effect
  • P1 is the prism power of lens 1
  • P2 is the prism power of lens 2

To calculate the differential prismatic effect, subtract the prism power of lens 2 from the prism power of lens 1.

What is Differential Prismatic Effect?

The differential prismatic effect refers to the difference in prismatic power between two lenses. This effect is important in the field of optometry and ophthalmology, as it can impact the visual comfort and clarity of individuals wearing corrective lenses. When there is a significant difference in the prismatic power of two lenses, it can cause visual discomfort, double vision, or other visual disturbances. Understanding and calculating the differential prismatic effect helps in designing lenses that provide optimal visual performance and comfort.

How to Calculate Differential Prismatic Effect?

The following steps outline how to calculate the Differential Prismatic Effect.

  1. First, determine the prism power of lens 1 (P1).
  2. Next, determine the prism power of lens 2 (P2).
  3. Finally, calculate the differential prismatic effect using the formula DPE = P1 – P2.
  4. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Prism power of lens 1 (P1) = 3 prism diopters

Prism power of lens 2 (P2) = 1.5 prism diopters