The Complete
Prism Guide
The definitive resource for Prentice's Rule, prism direction, and compounding effects.
The Complete Prism Guide
Generated on 1/11/2026
1. Prentice's Rule
P = (h * D) / 10
Induced prism is the product of decentration (mm) and lens power (D), divided by 10.
2. Base Direction (Plus Lenses)
Direction = Decentration
For plus lenses, the base direction is the same as the direction of decentration.
3. Base Direction (Minus Lenses)
Direction = Opposite
For minus lenses, the base direction is opposite to the direction of decentration.
Compounding vs. Cancelling
Horizontal Prism
Compounding
BO/BO or BI/BI
Cancelling
BO/BI
Prisms in the same direction (e.g., both Base Out) are compounding (additive).
Vertical Prism
Compounding
BU/BD
Cancelling
BU/BU or BD/BD
Vertical prisms in opposite directions (one Up, one Down) are compounding.
Advanced Concepts
Prism from Thickness
Δ = (100 * g * (n - 1)) / d
g = thickness diff, d = diameter, n = index. Useful for determining prism in a ground lens.
Resultant Prism
R = √(V² + H²)
Use the Pythagorean theorem to combine vertical and horizontal prism components.
Prism Splitting
Total / 2
Always split prism between both eyes for better weight balance and aesthetics when possible.
The 10-Minute Rule
Always double-check your horizontal prism compounding. Base IN / Base IN is additive because it moves the light in the same relative direction for both eyes (toward the nose). Base IN / Base OUT is subtractive.
The Optical Science of Prism
In optical physics, a prism is a transparent optical element with flat, polished surfaces that refract light. For an optician, prism is a critical tool used to correct binocular vision issues, such as strabismus or diplopia (double vision). When light passes through a prism, it is always bent toward the base of the prism, which causes the image to appear to shift toward the apex.
Understanding this fundamental behavior is essential for accurately dispensing corrective eyewear. Whether the prism is prescribed by a doctor or "induced" through poor centration, its effect on the patient's comfort and visual clarity cannot be overstated. Mastering Prentice's Rule allows you to calculate and control these effects with mathematical precision.
Clinical Applications
Prism is used in everyday practice for more than just correcting double vision:
Vertical Imbalance: Correcting the "differential prism" that occurs when a patient looks down to read through different powered lenses.
Convergence Insufficiency: Helping patients whose eyes have difficulty turning inward to focus on near objects.
Slab-Off Prism: A specialized grinding technique used to provide comfort for anisometropic patients at the near point.
A Master of Optical Math
Becoming proficient in prism calculations is a mark of a highly skilled optician. It demonstrates a deep understanding of geometric optics and a commitment to providing the highest level of patient care. Use this guide as a reference during your ABO studies and early days in the lab—eventually, these calculations will become second nature.
Still struggling with Prentice's Rule?
The best way to learn is by doing. Opterio has hundreds of prism calculation questions designed to make this second nature.