ABO NOCE Basic Opticianry Applying Prentice's Rule 2 — Questions and Answers
Question 1: A lens has a power of +3.00 D. A patient's pupil is decentered 4 mm below the optical center. How much vertical prism is induced?
- 1.20Δ base down
- 1.20Δ base up (Correct answer)
- 0.75Δ base down
- 0.75Δ base up
Correct answer: 1.20Δ base up
Δ = P × d = 3.00 × 0.4 = 1.20Δ; looking below the OC of a plus lens produces base-up prism because the prism base points toward the OC.
Question 2: Using Prentice's Rule, how much prism is induced when a -5.00 D lens is decentered 6 mm from the optical center?
- 2.50Δ
- 3.00Δ (Correct answer)
- 0.83Δ
- 30.00Δ
Correct answer: 3.00Δ
Δ = P × d = 5.00 × 0.6 cm = 3.00Δ.
Question 3: A patient requires 2.00Δ of prism. The available lens power is +4.00 D. How far must the lens be decentered to achieve this?
- 8 mm
- 2 mm
- 5 mm (Correct answer)
- 0.5 mm
Correct answer: 5 mm
d = Δ/P = 2.00/4.00 = 0.5 cm = 5 mm.
Question 4: Which unit for distance (d) must be used in Prentice's Rule formula Δ = P × d?
- Millimeters
- Centimeters (Correct answer)
- Meters
- Prism diopters
Correct answer: Centimeters
Prentice's Rule requires distance in centimeters; using millimeters without converting gives an answer 10 times too large.
Question 5: A frame PD is 68 mm and the patient's PD is 60 mm. The lens power is +2.50 D. How much horizontal prism is induced per eye if the decentration is split equally?
- 0.50Δ per eye
- 1.00Δ per eye (Correct answer)
- 2.00Δ per eye
- 2.50Δ per eye
Correct answer: 1.00Δ per eye
Total decentration = 8 mm; split = 4 mm = 0.4 cm per eye; Δ = 2.50 × 0.4 = 1.00Δ per eye.
Question 6: For a -2.00 D lens, the optical center is decentered 10 mm nasally in front of the right eye. What is the prismatic effect and base direction?
- 2.00Δ base in
- 2.00Δ base out (Correct answer)
- 0.20Δ base in
- 0.20Δ base out
Correct answer: 2.00Δ base out
Δ = 2.00 × 1.0 = 2.00Δ; for a minus lens decentered nasally in the right eye, prism base points toward the decentration = base out.
Question 7: A bifocal add of +2.50 D has its segment center 3 mm below the add's optical center. What is the vertical prismatic effect at the segment center?
- 0.75Δ base down (Correct answer)
- 0.75Δ base up
- 5.00Δ base up
- 0.50Δ base up
Correct answer: 0.75Δ base down
Δ = 2.50 × 0.3 = 0.75Δ; looking below the OC of a plus add produces base-down prism.
ABO NOCE Basic Opticianry Applying Prentice's Rule 2
A lens has a power of +3.00 D.
A patient's pupil is decentered 4 mm below the optical center.
How much vertical prism is induced?