ABO Ophthalmic Dispensing 2 — Questions and Answers
Question 1: A patient's prescription reads +2.50 -1.00 x 090. What is the power in the 180° meridian?
- +2.50 D
- +1.50 D (Correct answer)
- +3.50 D
- -1.00 D
Correct answer: +1.50 D
At 180°, the cylinder has no effect, so the power is the sphere (+2.50) plus the cylinder component at that meridian; the 90° axis means full cylinder effect at 180°, giving +2.50 + (-1.00) = +1.50 D.
Question 2: Which frame measurement represents the distance between the two lenses at their closest point?
- DBL (distance between lenses) (Correct answer)
- ED (effective diameter)
- A measurement
- Frame PD
Correct answer: DBL (distance between lenses)
DBL (distance between lenses) is the bridge measurement representing the narrowest gap between the two lenses.
Question 3: Progressive addition lenses typically have an unwanted astigmatism zone located:
- In the distance zone
- In the intermediate corridor
- In the near zone
- In the lateral peripheral areas (Correct answer)
Correct answer: In the lateral peripheral areas
Unwanted (peripheral) astigmatism in PALs is concentrated in the lateral areas on either side of the progressive corridor.
Question 4: When using a lensometer to verify a bifocal lens, the add power is determined by:
- Reading the near zone power directly
- Subtracting the distance power from the near zone power (Correct answer)
- Adding sphere and cylinder
- Measuring the segment height
Correct answer: Subtracting the distance power from the near zone power
Add power equals the net near zone power minus the net distance power measured by the lensometer.
Question 5: A high-minus lens will have the thinnest point at the:
- Optical center
- Edge of the lens (Correct answer)
- Top of the lens
- Nasal side
Correct answer: Edge of the lens
Minus lenses are thicker at the center and thinner at the edges, so the edge is the thinnest point.
Question 6: Which of the following best describes 'prism thinning' in lens fabrication?
- Adding base-up prism to flatten a lens
- Decentering the lens blank to reduce edge thickness on one side (Correct answer)
- Using a prism to measure lens power
- Grinding the lens to a uniform thickness
Correct answer: Decentering the lens blank to reduce edge thickness on one side
Prism thinning involves decentering the lens blank so that unwanted prism thins down the thicker edge of high-power lenses.
Question 7: A patient with a +3.00 D prescription is fitted with a frame where the pupillary distance requires 4mm of nasal decentration per eye. What horizontal prism effect results?
- 0.12 prism diopter BI per eye
- 1.2 prism diopters BI per eye (Correct answer)
- 1.2 prism diopters BO per eye
- 0.12 prism diopters BO per eye
Correct answer: 1.2 prism diopters BI per eye
Using Prentice's Rule: P = D × d = 3.00 × 0.4 cm = 1.2 prism diopters; nasal decentration with a plus lens creates base-in prism.
ABO Ophthalmic Dispensing 2
A patient's prescription reads +2.50 -1.00 x 090.
What is the power in the 180° meridian?