NOCE Ophthalmic Optics and Formulas 5 — Questions and Answers
Question 1: The standard fitting height for a flat-top 28 bifocal segment is measured from:
- The frame top to the bifocal line
- The lowest point of the pupil to the top of the bifocal segment (Correct answer)
- The lens bottom to the top of the segment
- The geometric center of the lens to the segment top
Correct answer: The lowest point of the pupil to the top of the bifocal segment
Segment height is measured from the lowest limbal margin of the pupil down to the top of the bifocal segment.
Question 2: Spectacle magnification (SM) is the product of which two factors?
- Power factor and shape factor (Correct answer)
- Index factor and thickness factor
- Abbe value and base curve
- Pantoscopic tilt and vertex distance
Correct answer: Power factor and shape factor
SM = Power Factor × Shape Factor; power factor depends on back vertex power and vertex distance, shape factor on front curve and thickness.
Question 3: A near addition of +2.50 D allows a patient to focus at what working distance?
- 20 cm
- 25 cm
- 40 cm (Correct answer)
- 50 cm
Correct answer: 40 cm
Working distance = 1 / add power = 1 / 2.50 = 0.40 m = 40 cm.
Question 4: Monocular PD for bifocal segment centration should be measured with the patient:
- Looking straight ahead at distance fixation (Correct answer)
- Looking down at their near reading material
- Fixating a target 6 meters away with one eye covered
- Looking at the dispenser's nose tip at arm's length
Correct answer: Looking straight ahead at distance fixation
Distance monocular PD is measured with the patient looking at a distance target, ensuring proper optical center placement for the distance portion.
Question 5: Image jump at the top of a bifocal segment is determined by:
- The add power multiplied by the distance from the segment top to the segment's optical center (Correct answer)
- The distance Rx power multiplied by the total segment height
- Prentice's rule applied to the distance Rx at the segment line
- The Abbe value of the segment material and the add power
Correct answer: The add power multiplied by the distance from the segment top to the segment's optical center
Image jump = add power × distance from segment top to segment optical center; for flat-tops the OC is below the top, causing jump.
Question 6: The minimum blank size (MBS) formula for lens ordering is MBS =
- Effective diameter (ED) + 2 mm safety margin + 2 × decentration per side (Correct answer)
- Frame PD minus the patient's distance PD
- DBL + frame width ÷ 2
- Segment height plus pantoscopic tilt compensation
Correct answer: Effective diameter (ED) + 2 mm safety margin + 2 × decentration per side
MBS = ED + 2 × decentration + safety margin (typically 2 mm) to ensure the blank covers the entire edged lens.
Question 7: Slab-off prism is used to correct vertical imbalance in anisometropic bifocal wearers. The amount of slab-off is determined by:
- Prentice's rule applied to the difference in add powers at the near reference point (Correct answer)
- The spherical equivalent difference between the two eyes at distance
- The segment height difference between the two lenses
- The index of refraction difference between the two lens materials
Correct answer: Prentice's rule applied to the difference in add powers at the near reference point
Slab-off corrects the differential vertical prism induced by the add; calculated using Prentice's rule on the add power difference at the reading level.
NOCE Ophthalmic Optics and Formulas 5
The standard fitting height for a flat-top 28 bifocal segment is measured from: