Free ABO-NOCE Applying Prentice's Rule Questions and Answers — Questions and Answers
Question 1: A patient with a prescription of -4.00 DS looks 5 mm above the optical center. How much prism is induced and in what direction?
- 2.0Δ Base Up
- 2.0Δ Base Down (Correct answer)
- 0.8Δ Base Up
- 0.8Δ Base Down
Correct answer: 2.0Δ Base Down
Prentice's Rule is P = cF, where 'c' is in cm. P = (0.5 cm) * (4.00 D) = 2.0Δ. Looking up through a minus lens induces Base Down (BD) prism. This can be remembered with the acronym BUDA: Base Up Down and Away (from the OC).
Question 2: A lens with a power of +6.00 D is decentered 3 mm nasally. What is the resulting prism?
- 1.8Δ Base Out
- 2.0Δ Base In
- 1.8Δ Base In (Correct answer)
- 2.0Δ Base Out
Correct answer: 1.8Δ Base In
Using Prentice's Rule: P = cF = (0.3 cm) * (6.00 D) = 1.8Δ. Decentering a plus lens nasally moves the optical center nasally. To look through the OC, the eye would have to turn in, so the prism is Base In (BI).
Question 3: A patient's PD is 64 mm, but their glasses were made with the optical centers at 60 mm. If the Rx is -5.00 DS in both eyes, what is the total induced prism?
- 2.0Δ Base In
- 1.0Δ Base Out
- 1.0Δ Base In
- 2.0Δ Base Out (Correct answer)
Correct answer: 2.0Δ Base Out
The total decentration is 4 mm, or 2 mm per eye. The OCs are too narrow, so they are decentered nasally. P = cF = (0.2 cm) * (5.00 D) = 1.0Δ per eye. For a minus lens, nasal decentration creates Base Out prism. The total is 1.0Δ BO + 1.0Δ BO = 2.0Δ BO.
Question 4: How much must a -10.00 D lens be decentered to produce 3.0Δ of prism?
- 1.5 mm
- 3 mm (Correct answer)
- 3.3 mm
- 30 mm
Correct answer: 3 mm
Rearranging Prentice's Rule gives c = P / F. So, c = 3.0Δ / 10.00 D = 0.3 cm. To express the answer in millimeters, multiply by 10. 0.3 cm * 10 = 3 mm.
Question 5: A patient with Rx OD: +2.00 DS, OS: +4.00 DS looks 10 mm down to read. What is the vertical imbalance?
- 1.0Δ
- 2.0Δ (Correct answer)
- 4.0Δ
- 6.0Δ
Correct answer: 2.0Δ
First, calculate the prism for each eye. OD: P = (1.0 cm) * (2.00 D) = 2.0Δ BD. OS: P = (1.0 cm) * (4.00 D) = 4.0Δ BD. Since both bases are in the same direction, the imbalance is the difference between them: 4.0Δ - 2.0Δ = 2.0Δ.
Question 6: In the formula P = cF, the variable 'c' must be expressed in which unit?
- Millimeters
- Inches
- Centimeters (Correct answer)
- Meters
Correct answer: Centimeters
For Prentice's Rule to work correctly and yield an answer in prism diopters, the distance of decentration ('c') must be in centimeters. The lens power ('F') must be in diopters.
Question 7: A patient wearing +3.50 D reading glasses holds their reading material 4 cm to the right of center. How much prism is induced horizontally?
- 1.4Δ Base Out
- 14.0Δ Base Out
- 0.875Δ Base Out
- 0Δ (Correct answer)
Correct answer: 0Δ
This question is a distractor. Prentice's rule applies when the *eye* moves away from the optical center, not when the object being viewed moves. As long as the patient looks straight through the optical center of the lens, no prism is induced, regardless of where the object is.
