ABO Ophthalmic Dispensing 4 — Questions and Answers
Question 1: When a patient complains of 'jack-in-the-box' phenomenon with new bifocals, the most likely cause is:
- Incorrect optical center height
- Segment set too high (Correct answer)
- Excessive pantoscopic tilt
- Incorrect pupillary distance
Correct answer: Segment set too high
The 'jack-in-the-box' effect occurs when the bifocal segment is fitted too high, causing objects to pop in and out of view abruptly at the segment line.
Question 2: The term 'effective diameter' (ED) of a lens refers to:
- The horizontal width (A measurement) of the lens
- The diameter of the smallest circle enclosing the entire lens shape (Correct answer)
- The vertical height (B measurement) of the lens
- The distance from lens center to the farthest edge
Correct answer: The diameter of the smallest circle enclosing the entire lens shape
ED is the diameter of the smallest circle that completely encloses the lens shape, used to determine minimum blank size needed.
Question 3: A patient's single vision lenses show prismatic displacement of images. Which instrument is used to measure this induced prism?
- Pupillometer
- Lensometer (lensmeter) (Correct answer)
- Keratometer
- Geneva lens clock
Correct answer: Lensometer (lensmeter)
The lensometer (focimeter) is used to measure lens power and to quantify prismatic effect by locating the optical center relative to the pupil position.
Question 4: For a frame with A=52, DBL=18, the standard PD (frame PD) equals:
- 52 mm
- 70 mm (Correct answer)
- 34 mm
- 88 mm
Correct answer: 70 mm
Frame PD = A + DBL = 52 + 18 = 70 mm; this represents the distance from one lens center to the other.
Question 5: A base curve of +6.00 D is selected for a +4.00 D lens. This is best described as a:
- Corrected curve (best-form) lens (Correct answer)
- Deep curve lens
- Flat curve lens
- Lenticular design
Correct answer: Corrected curve (best-form) lens
A corrected curve (best-form) lens uses a specific base curve chosen to minimize aberrations for a given prescription power.
Question 6: Polycarbonate lenses require a hard coat because the material itself is:
- Too brittle without it
- Highly scratch-resistant natively
- Relatively soft and scratch-prone (Correct answer)
- Hydrophilic without treatment
Correct answer: Relatively soft and scratch-prone
Polycarbonate is inherently soft and scratches easily, so a factory-applied hard coat is essential to provide adequate scratch resistance.
Question 7: Which measurement is used to set the height of the progressive lens fitting cross on the patient's face?
- Monocular PD
- Segment height (fitting height) (Correct answer)
- DBL
- Vertex distance
Correct answer: Segment height (fitting height)
The fitting height (measured from the bottom of the frame to the pupil center) is used to position the progressive lens fitting cross at the pupil level.
ABO Ophthalmic Dispensing 4
When a patient complains of 'jack-in-the-box' phenomenon with new bifocals, the most likely cause is: