ABO Optics and Refraction 2 — Questions and Answers
Question 1: An object is placed 50 cm in front of a +4.00 D thin lens. Where is the image formed?
- 100 cm behind the lens
- 25 cm behind the lens
- 50 cm behind the lens (Correct answer)
- 50 cm in front of the lens (virtual)
Correct answer: 50 cm behind the lens
Object vergence is -2.00 D, so image vergence is -2.00 + 4.00 = +2.00 D, forming a real image 50 cm behind the lens.
Question 2: A -5.00 D spectacle lens is decentered 3 mm from the visual axis. Per Prentice's rule, how much prism is induced?
- 15 prism diopters
- 0.6 prism diopters
- 3.0 prism diopters
- 1.5 prism diopters (Correct answer)
Correct answer: 1.5 prism diopters
Prentice's rule: prism = decentration (cm) x power (D) = 0.3 x 5 = 1.5 prism diopters.
Question 3: What is the spherical equivalent of +2.00 -3.00 x 090?
- -1.00 D
- +0.50 D (Correct answer)
- -0.50 D
- +3.50 D
Correct answer: +0.50 D
Spherical equivalent = sphere + half the cylinder = +2.00 + (-1.50) = +0.50 D.
Question 4: What is the correct plus-cylinder transposition of +1.00 -2.00 x 180?
- +3.00 -2.00 x 090
- -1.00 +2.00 x 180
- -1.00 +2.00 x 090 (Correct answer)
- +1.00 +2.00 x 090
Correct answer: -1.00 +2.00 x 090
Add sphere and cylinder for the new sphere, flip the cylinder sign, and rotate the axis 90 degrees.
Question 5: A patient is corrected with a +10.00 D spectacle lens at a 12 mm vertex distance. What contact lens power is approximately required?
- +10.00 D
- +11.25 D (Correct answer)
- +8.75 D
- +12.50 D
Correct answer: +11.25 D
Effective power = F/(1 - dF) = 10/(1 - 0.012 x 10) ≈ +11.36 D, so about +11.25 D.
Question 6: According to Knapp's law, axial ametropia corrected by a spectacle lens placed at the anterior focal point of the eye produces what retinal image size?
- Smaller than an emmetropic eye
- Variable depending on pupil size
- Same as that of an emmetropic eye (Correct answer)
- Larger than an emmetropic eye
Correct answer: Same as that of an emmetropic eye
Knapp's law states that a lens at the anterior focal point (~15 mm) yields an emmetropic-sized retinal image in axial ametropia.
Question 7: When refining cylinder axis with a Jackson cross cylinder in a minus-cylinder refraction, which way should the axis be rotated?
- Toward the plus axis of the preferred cross-cylinder position
- Toward the meridian of least corneal power
- Always 90 degrees from the current axis
- Toward the minus axis of the preferred cross-cylinder position (Correct answer)
Correct answer: Toward the minus axis of the preferred cross-cylinder position
In minus-cylinder technique, rotate the trial cylinder axis toward the red (minus) dot of the preferred flip position.
ABO Optics and Refraction 2
An object is placed 50 cm in front of a +4.00 D thin lens.
Where is the image formed?