ABO Ametropia/Ophthalmic Optics 5 — Questions and Answers
Question 1: Anisometropia is clinically significant when the difference in refractive error between the two eyes exceeds approximately:
- 0.50 D
- 1.00 D (Correct answer)
- 2.50 D
- 5.00 D
Correct answer: 1.00 D
A difference of 1.00 D or more between eyes is generally considered clinically significant anisometropia that may affect binocular vision.
Question 2: What is the prismatic effect at a point 8 mm below the optical center of a -5.00 D lens (Prentice's rule)?
- 2.5Δ base-up
- 4.0Δ base-down
- 4.0Δ base-up (Correct answer)
- 2.5Δ base-down
Correct answer: 4.0Δ base-up
P = c × F = 0.8 cm × 5.00 = 4.0Δ; a minus lens produces base-up prism in the inferior portion (base toward optical center).
Question 3: Which of the following best describes 'lenticular myopia'?
- Myopia due to excessive axial length
- Myopia caused by increased curvature or index of the crystalline lens (Correct answer)
- Myopia from corneal steepening alone
- Myopia present only during near work
Correct answer: Myopia caused by increased curvature or index of the crystalline lens
Lenticular myopia results from optical changes within the crystalline lens, such as increased curvature, thickness, or index of refraction.
Question 4: The power of a spherical refracting surface is calculated using which formula?
- P = (n' - n) / r (Correct answer)
- P = n × r
- P = (n + n') / r
- P = n' / (n × r)
Correct answer: P = (n' - n) / r
The power of a single refracting surface is P = (n' - n) / r, where n' is the exit medium, n is the entry medium, and r is the radius in meters.
Question 5: A patient with high myopia (-10.00 D) is best served by high-index lens materials primarily because:
- They improve UV protection
- They reduce lens thickness and weight (Correct answer)
- They increase the spectacle magnification
- They eliminate chromatic aberration
Correct answer: They reduce lens thickness and weight
Higher index materials allow the same power to be achieved with less curvature, resulting in thinner and lighter lenses for high prescriptions.
Question 6: Slab-off (bicentric grinding) is used in spectacle lenses to compensate for:
- Vertical prism imbalance at near in anisometropic patients (Correct answer)
- Horizontal prism for esophoria
- Chromatic aberration in high-power lenses
- Distortion from high wrap angles
Correct answer: Vertical prism imbalance at near in anisometropic patients
Slab-off creates vertical prism in one lens to equalize the differential vertical prismatic effect that occurs at the near reading position in anisometropia.
Question 7: Which of the following is the correct definition of the 'far point' in hyperopia?
- A real point located in front of the eye
- A virtual point located behind the eye (Correct answer)
- The point at which accommodation is fully relaxed at distance
- The midpoint of the accommodative range
Correct answer: A virtual point located behind the eye
In hyperopia, the far point is a virtual conjugate point located behind the eye because the relaxed eye's focal point falls behind the retina.
ABO Ametropia/Ophthalmic Optics 5
Anisometropia is clinically significant when the difference in refractive error between the two eyes exceeds approximately: