ABO Ametropia/Ophthalmic Optics 4 — Questions and Answers
Question 1: A patient requires +2.50 D at the spectacle plane (12 mm vertex distance). What contact lens power is needed?
- +2.44 D (Correct answer)
- +2.57 D
- +2.50 D
- +2.37 D
Correct answer: +2.44 D
F_contact = F_spec / (1 + d × F_spec) = 2.50 / (1 + 0.012 × 2.50) = 2.50 / 1.03 ≈ +2.43 D, rounded to +2.44 D.
Question 2: Oblique astigmatism refers to astigmatism with principal meridians oriented at:
- 0 and 90 degrees
- 45 and 135 degrees
- Between 30–60 and 120–150 degrees (Correct answer)
- Any axis other than 180 and 090
Correct answer: Between 30–60 and 120–150 degrees
Oblique astigmatism is conventionally defined as astigmatism with the principal meridians between 30–60 degrees and 120–150 degrees.
Question 3: The Maddox rod test is used clinically to measure:
- The amount of astigmatism
- Heterophoria and vergence deviation (Correct answer)
- Accommodative amplitude
- Anisometropia
Correct answer: Heterophoria and vergence deviation
The Maddox rod converts a point source into a streak, dissociating the eyes to reveal and measure phoria or tropia.
Question 4: Which lens type is most effective at reducing oblique astigmatic error for off-axis gaze?
- Plano lens
- Corrected curve (best form) lens (Correct answer)
- Flat-front lens
- Biconcave lens
Correct answer: Corrected curve (best form) lens
Corrected curve (best form) lenses are designed with base curves that minimize oblique astigmatism during off-axis viewing.
Question 5: A patient's spectacle magnification is greater than 1.0. This indicates the patient is:
- Myopic
- Emmetropic
- Hyperopic (Correct answer)
- Presbyopic
Correct answer: Hyperopic
Spectacle magnification >1.0 means the retinal image is larger than in the standard emmetropic eye, which occurs with plus (hyperopic) lenses.
Question 6: The Jackson cross cylinder (JCC) is used to refine which components of the refraction?
- Sphere power only
- Cylinder power and axis only (Correct answer)
- Add power and base curve
- Sphere, cylinder, and axis
Correct answer: Cylinder power and axis only
The JCC is specifically designed to refine the cylindrical axis and power without changing the spherical equivalent.
Question 7: In the reduced eye model, what is the approximate refractive index used for the ocular media?
- 1.000
- 1.336 (Correct answer)
- 1.523
- 1.400
Correct answer: 1.336
The reduced eye model uses a single refractive index of approximately 1.336 to represent the combined ocular media.
ABO Ametropia/Ophthalmic Optics 4
A patient requires +2.50 D at the spectacle plane (12 mm vertex distance).
What contact lens power is needed?