ABO Optics and Refraction 2

Question 1

An object is placed 50 cm in front of a +4.00 D thin lens. Where is the image formed?

Accepted Answer

50 cm behind the lens

Answer

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?

Accepted Answer

1.5 prism diopters

Answer

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?

Accepted Answer

+0.50 D

Answer

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?

Accepted Answer

-1.00 +2.00 x 090

Answer

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?

Accepted Answer

+11.25 D

Answer

Effective power = F/(1 - dF) = 10/(1 - 0.012 x 10) ≈ +11.36 D, so about +11.25 D.

ABO - American Board of Ophthalmology Practice Test