Ophthalmic Dispensing Flashcards

A patient's prescription reads +2.50 -1.00 x 090. What is the power in the 180° meridian?

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.

Which frame measurement represents the distance between the two lenses at their closest point?

Answer: DBL (distance between lenses)

DBL (distance between lenses) is the bridge measurement representing the narrowest gap between the two lenses.

Progressive addition lenses typically have an unwanted astigmatism zone located:

Answer: In the lateral peripheral areas

Unwanted (peripheral) astigmatism in PALs is concentrated in the lateral areas on either side of the progressive corridor.

When using a lensometer to verify a bifocal lens, the add power is determined by:

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.

A high-minus lens will have the thinnest point at the:

Answer: Edge of the lens

Minus lenses are thicker at the center and thinner at the edges, so the edge is the thinnest point.

Which of the following best describes 'prism thinning' in lens fabrication?

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.

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?

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.