Ametropia/Ophthalmic Optics Flashcards

A patient has a prescription of -3.00 -1.50 x 090. What is the power in the 180-degree meridian?

Answer: -4.50 D

In the 180-degree meridian (perpendicular to axis 090), the full sphere plus cylinder power applies: -3.00 + (-1.50) = -4.50 D.

Which type of astigmatism has one focal line in front of the retina and one behind it?

Answer: Mixed astigmatism

Mixed astigmatism has one principal meridian focused anterior to the retina (myopic) and the other posterior (hyperopic).

A +2.00 D lens is placed 12 mm in front of the corneal plane. What is the equivalent power at the corneal plane?

Answer: +2.05 D

Using F' = F / (1 - dF) = 2.00 / (1 - 0.012 × 2.00) = 2.00 / 0.976 ≈ +2.05 D.

The circle of least confusion in a Sturm's conoid is located:

Answer: Midway between the two focal lines

The circle of least confusion is the round cross-section of Sturm's conoid located equidistant between the two focal lines.

A patient with 4.00 D of axial myopia has an eye axial length that is approximately how much longer than emmetropic?

Answer: 1.4 mm

Each diopter of axial myopia corresponds to roughly 0.35 mm of excess axial length, so 4.00 D ≈ 1.4 mm.

Which prism effect occurs when a myopic patient looks through the upper portion of a single-vision minus lens while reading?

Answer: Base-up prism

A minus lens creates base-up prismatic effect in its upper portion due to Prentice's rule, which can cause reading difficulties.

Which refractive condition results from a cornea with greater curvature in the vertical meridian than the horizontal?

Answer: With-the-rule astigmatism

With-the-rule astigmatism occurs when the vertical corneal meridian is steeper, requiring a minus cylinder axis at 180 degrees.