ABO NOCE Basic Opticianry Ophthalmic Optics and Principles 4 — Questions and Answers
Question 1: Using Prentice's Rule, what is the prismatic effect at a point 8 mm from the optical center of a +5.00 D lens?
- 2Δ
- 4Δ (Correct answer)
- 8Δ
- 40Δ
Correct answer: 4Δ
Prentice's Rule: Prism (Δ) = d(cm) × F = 0.8 × 5.00 = 4.00Δ.
Question 2: What is the purpose of the 'crossed cylinder' test in refraction?
- To determine the sphere power
- To refine cylinder axis and power (Correct answer)
- To measure pupillary distance
- To detect malingering
Correct answer: To refine cylinder axis and power
The Jackson crossed cylinder (JCC) is used to precisely refine both the axis and power of the correcting cylinder during subjective refraction.
Question 3: What is 'vergence' in the context of ophthalmic optics?
- The color of a lens tint
- The curvature of wavefronts of light (Correct answer)
- The thickness of a lens center
- The frame adjustment angle
Correct answer: The curvature of wavefronts of light
Vergence describes the degree of convergence or divergence of a wavefront of light, measured in diopters.
Question 4: A lens with a refractive index of 1.74 compared to 1.50 will be:
- Thicker but lighter
- Thinner and lighter (Correct answer)
- Thicker and heavier
- The same thickness
Correct answer: Thinner and lighter
Higher index materials bend light more strongly, allowing thinner lens profiles; modern high-index materials are also less dense, making lenses lighter.
Question 5: What is the effect of increasing pantoscopic tilt on the effective power of a lens?
- It reduces sphere power only
- It induces plus sphere and minus cylinder (Correct answer)
- It has no effect on prescription
- It reverses the cylinder axis
Correct answer: It induces plus sphere and minus cylinder
Increasing pantoscopic tilt introduces oblique astigmatism, effectively adding plus sphere power and a minus cylinder component to the lens's effective prescription.
Question 6: Which formula correctly expresses the lensmaker's equation relationship for a thin lens?
- P = (n-1)(1/R1 - 1/R2) (Correct answer)
- P = n(R1 + R2)
- P = (n+1)/(R1 × R2)
- P = R1 - R2 / n
Correct answer: P = (n-1)(1/R1 - 1/R2)
The lensmaker's equation is P = (n-1)(1/R1 - 1/R2), where n is the refractive index and R1, R2 are the radii of curvature of the two surfaces.
Question 7: What is the spherical equivalent of -2.00 -1.00 × 180?
- -2.00 D
- -2.50 D (Correct answer)
- -3.00 D
- -1.50 D
Correct answer: -2.50 D
Spherical equivalent = sphere + (cylinder/2) = -2.00 + (-1.00/2) = -2.00 + (-0.50) = -2.50 D.
ABO NOCE Basic Opticianry Ophthalmic Optics and Principles 4
Using Prentice's Rule, what is the prismatic effect at a point 8 mm from the optical center of a +5.00 D lens?