ABO Ophthalmic Dispensing 3 — Questions and Answers
Question 1: Which lens material has the highest Abbe value among common ophthalmic materials?
- Polycarbonate
- Trivex
- CR-39 (plastic) (Correct answer)
- 1.74 high-index
Correct answer: CR-39 (plastic)
CR-39 has an Abbe value of approximately 58, the highest among common ophthalmic lens materials, meaning least chromatic aberration.
Question 2: In frame adjustment, the 'face-form angle' (wrap) primarily affects which aspect of the fit?
- Vertex distance
- Pantoscopic tilt
- The degree to which the frame curves to follow the face (Correct answer)
- Temple length
Correct answer: The degree to which the frame curves to follow the face
Face-form angle (wrap) describes how much the frame front curves horizontally to follow the contour of the face.
Question 3: A prescription with cylinder power is written in minus cylinder form as -2.00 -1.50 x 045. What is the equivalent plus cylinder form?
- -3.50 +1.50 x 135 (Correct answer)
- +2.00 +1.50 x 135
- -0.50 +1.50 x 135
- -3.50 +1.50 x 045
Correct answer: -3.50 +1.50 x 135
To transpose: new sphere = algebraic sum of sphere and cylinder (-2.00 + -1.50 = -3.50), new cylinder = same magnitude with opposite sign (+1.50), new axis = 90° away (045 + 90 = 135).
Question 4: The segment height for a flat-top bifocal is measured from the:
- Top of the lens to the top of the segment
- Bottom of the frame to the top of the segment (Correct answer)
- Optical center to the segment line
- Pupil center to the bottom of the lens
Correct answer: Bottom of the frame to the top of the segment
Segment height is measured from the lowest point of the frame's inner groove (bottom) up to the top of the bifocal segment.
Question 5: Which of the following coatings primarily reduces internal lens reflections that reach the wearer's eyes from behind?
- UV coating
- Anti-reflective (AR) coating (Correct answer)
- Mirror coating
- Hard coat
Correct answer: Anti-reflective (AR) coating
AR coating reduces reflections on both front and back lens surfaces, including light entering from behind that would otherwise reflect into the wearer's eyes.
Question 6: A pantoscopic tilt of 8–12° is recommended because it:
- Increases vertex distance
- Aligns the lens optical axis closer to the line of sight for reading (Correct answer)
- Reduces lens weight
- Improves UV protection
Correct answer: Aligns the lens optical axis closer to the line of sight for reading
Pantoscopic tilt angles the bottom of the lens closer to the cheek, aligning the optical axis more closely with the downward gaze direction used for near vision.
Question 7: Which type of bifocal segment provides the widest near reading area?
- Round segment (22mm)
- Flat-top 28 segment
- Flat-top 35 segment
- Executive (full-width) segment (Correct answer)
Correct answer: Executive (full-width) segment
The executive (Franklin) bifocal runs the full width of the lens, providing the widest possible near reading zone.
ABO Ophthalmic Dispensing 3
Which lens material has the highest Abbe value among common ophthalmic materials?