COT Optics and Lensometry 3 — Questions and Answers
Question 1: What is the add power of a bifocal if the distance power is -2.00 sph and the near power reads -0.50 sph?
- +1.00 D
- +1.50 D (Correct answer)
- +2.50 D
- -1.50 D
Correct answer: +1.50 D
Add power = near power minus distance power: (-0.50) - (-2.00) = +1.50 D.
Question 2: Which type of lens corrects presbyopia by providing a gradual change in power from distance to near without a visible segment line?
- Flat-top bifocal
- Executive bifocal
- Progressive addition lens (Correct answer)
- Lenticular lens
Correct answer: Progressive addition lens
Progressive addition lenses (PALs) provide a corridor of increasing plus power from the distance zone to the near zone with no visible line.
Question 3: What is the focal length of a +4.00 D lens?
- 4 meters
- 0.25 meters (Correct answer)
- 0.4 meters
- 2.5 meters
Correct answer: 0.25 meters
Focal length (f) = 1 / power in diopters = 1 / 4.00 = 0.25 meters.
Question 4: Chromatic aberration in optics occurs because different wavelengths of light are refracted at slightly different angles, causing what visual effect?
- Monocular diplopia
- Color fringing around objects (Correct answer)
- Reduced contrast sensitivity
- Increased depth of field
Correct answer: Color fringing around objects
Chromatic aberration separates colors at the focal point, producing colored fringes around high-contrast edges.
Question 5: A lens has sphere power of -3.00 D and cylinder power of -1.00 D. What is the power in the meridian of the cylinder axis?
- -4.00 D
- -3.00 D (Correct answer)
- -2.00 D
- -1.00 D
Correct answer: -3.00 D
The cylinder axis meridian carries only the sphere power; the cylinder power is added in the meridian 90° away from the axis.
Question 6: What is the purpose of neutralizing a lens when performing manual lensometry without a lensmeter?
- To determine the lens color tint
- To find the lens power by canceling it with known trial lenses (Correct answer)
- To measure the lens thickness
- To evaluate the antireflective coating
Correct answer: To find the lens power by canceling it with known trial lenses
Neutralization involves adding trial lenses of opposite power until no apparent movement of an object is seen, indicating the total power.
Question 7: When reading the lensmeter, a technician finds the sphere power but the mires cannot be focused to a single clear line simultaneously. What does this indicate?
- The lens has prism
- The lens has cylinder (astigmatic power) (Correct answer)
- The lens is scratched
- The lensmeter needs calibration
Correct answer: The lens has cylinder (astigmatic power)
Inability to bring all mires into simultaneous focus indicates astigmatic (cylindrical) power in the lens.
COT Optics and Lensometry 3
What is the add power of a bifocal if the distance power is -2.00 sph and the near power reads -0.50 sph?