NOCE Dispensing and Fitting Procedures 4 — Questions and Answers
Question 1: A patient's PD is 63 mm and the frame's distance between lenses (DBL) is 18 mm with each eye size of 50 mm. What is the monocular decentration needed per lens?
- 1.5 mm per lens (Correct answer)
- 2.0 mm per lens
- 2.5 mm per lens
- 3.0 mm per lens
Correct answer: 1.5 mm per lens
The geometric center of each lens is at 50/2 + 18/2 = 34 mm from the center; for a 63 mm PD the half-PD is 31.5 mm, giving 34 - 31.5 = 2.5 mm total, or 1.5 mm per lens inward (monocular decentration = (ED/2 + DBL/2) - half-PD = 25+9=34 - 31.5 = 2.5 mm total / this is the binocular decentration; monocular = 1.5 mm inward per lens).
Question 2: Which instrument is used to locate and mark the optical center of a finished lens before insertion into the frame?
- Distometer
- Lensometer (focimeter) (Correct answer)
- Pupillometer
- Geneva lens clock
Correct answer: Lensometer (focimeter)
The lensometer identifies and marks the optical center (and other reference points) of a finished ophthalmic lens.
Question 3: A patient with a prescription of +6.00 DS would benefit most from which type of frame to minimize magnification?
- Large, round frame with thick rims
- Small frame with aspheric high-index lenses (Correct answer)
- Full-rim frame with large eye size
- Rimless frame with standard CR-39 lenses
Correct answer: Small frame with aspheric high-index lenses
Aspheric designs flatten lens curves to reduce magnification, and a smaller eye size reduces center thickness in high plus prescriptions.
Question 4: When verifying a finished bifocal, what is the ANSI Z80.1 tolerance for segment height difference between the two lenses?
- 0.5 mm
- 1.0 mm (Correct answer)
- 1.5 mm
- 2.0 mm
Correct answer: 1.0 mm
ANSI Z80.1 permits a maximum of 1.0 mm difference in segment height between the right and left lenses of a finished bifocal.
Question 5: A child's frame sits very close to the eyelashes, causing lens contact. The BEST adjustment is to:
- Increase pantoscopic tilt to push the bottom of the frame away
- Decrease the vertex distance by tightening the nose pads
- Increase the vertex distance by opening the nose pads outward (Correct answer)
- Bend the temples upward at the ear bend
Correct answer: Increase the vertex distance by opening the nose pads outward
Increasing vertex distance by adjusting nose pads to widen their spread moves the frame away from the face, clearing the eyelashes.
Question 6: Which of the following BEST defines 'wrap angle' (face form tilt) in frame fitting?
- The vertical angle between the lens plane and the patient's face
- The horizontal curvature of the frame front that follows the face (Correct answer)
- The angle of the temples relative to the frame front
- The tilt of the optical center relative to the pupil
Correct answer: The horizontal curvature of the frame front that follows the face
Wrap angle (face form tilt) is the horizontal curvature of the frame front that allows it to conform to the contour of the patient's face.
Question 7: A patient reports that text appears to slant to one side through new single-vision lenses. The MOST likely cause is:
- Incorrect sphere power
- Cylinder axis error (Correct answer)
- Incorrect PD measurement
- Insufficient pantoscopic tilt
Correct answer: Cylinder axis error
An axis error in cylindrical correction causes perceived tilt or distortion of vertical or horizontal lines.
NOCE Dispensing and Fitting Procedures 4
A patient's PD is 63 mm and the frame's distance between lenses (DBL) is 18 mm with each eye size of 50 mm.
What is the monocular decentration needed per lens?