ABO NOCE Basic Opticianry Spectacle Dispensing and Fitting Questions and Answers 2 — Questions and Answers
Question 1: What is the pantoscopic tilt and what is the standard recommended angle?
- The vertical angle of the frame front; typically 8–12° (Correct answer)
- The horizontal angle of the frame; typically 0–5°
- The wrap angle of the frame; typically 15–20°
- The angle of the temples; typically 5–10°
Correct answer: The vertical angle of the frame front; typically 8–12°
Pantoscopic tilt is the angle of the frame front in the vertical plane (bottom tilted toward the face). The standard is approximately 8–12 degrees.
Pantoscopic tilt (or pantoscopic angle) is the forward tilt of the spectacle frame in the vertical plane, where the bottom of the lens is closer to the face than the top. The standard recommended angle is 8–12°. This tilt is important because it aligns the optical axis of the lens more perpendicular to the line of sight for near vision tasks. Excessive pantoscopic tilt induces unwanted cylinder in the line of sight. When adjusting pantoscopic tilt, for every 2° of tilt, the optical center should be moved 1 mm downward.
Question 2: When measuring monocular pupillary distance (PD), what is the correct procedure?
- Measure from the center of the pupil to the center of the nose bridge for each eye separately (Correct answer)
- Measure from pupil center to pupil center with the patient fixating a distant target
- Measure from the outer canthus to the pupil center
- Measure from the lens optical center to the pupil
Correct answer: Measure from the center of the pupil to the center of the nose bridge for each eye separately
Monocular PD is measured from the center of each pupil to the center of the nose bridge, giving separate right and left measurements.
Monocular pupillary distance (PD) measures the distance from the center of each pupil to the midpoint of the nose bridge independently for right and left eyes. This is important because patients are not always symmetric — the right and left PDs may differ. Total binocular PD = right monocular PD + left monocular PD. Monocular PDs are especially critical for progressive lenses and strong prescriptions where optical center placement must be precise. Standard monocular PD measurement is done with a PD ruler or pupilometer while the patient fixates a target at distance (approximately 6 meters or fixating the examiner's eye).
Question 3: What is the vertex distance and why does it matter for prescriptions over ±4.00 D?
- The distance from the back lens surface to the cornea; it affects effective lens power at the eye (Correct answer)
- The distance from the optical center to the pupil; it determines PD accuracy
- The distance from the frame front to the temples; it affects frame fit
- The distance from the top of the frame to the pupil center; it determines seg height
Correct answer: The distance from the back lens surface to the cornea; it affects effective lens power at the eye
Vertex distance is the distance from the back surface of the lens to the cornea. For powers over ±4.00 D, this distance significantly affects the effective power at the eye.
Vertex distance is the space between the posterior surface of the spectacle lens and the anterior surface of the cornea, typically 12–14 mm. For low to moderate prescriptions (under ±4.00 D), vertex distance has minimal effect. For stronger prescriptions, however, moving a lens closer to or farther from the eye changes its effective power significantly. The vertex power formula is: D_eff = D_lens / (1 - d × D_lens), where d is vertex distance in meters. This is why the vertex distance used during refraction should be documented on the prescription if over ±4.00 D, so the optician can compensate when dispensing.
Question 4: The facial measurement 'DBL' (distance between lenses) refers to:
- The horizontal distance between the inner edges of the two lenses (Correct answer)
- The distance from pupil center to pupil center
- The width of the nose bridge of the frame
- The distance between the optical centers of the two lenses
Correct answer: The horizontal distance between the inner edges of the two lenses
DBL is the distance between lenses — the clear horizontal gap between the inner (nasal) edges of the two lenses in the frame.
DBL (Distance Between Lenses, also called the bridge size in frame sizing) is the horizontal measurement of the space between the inner edges of the two lenses, measured at the narrowest point. It is part of the standard frame sizing notation: Eye Size / DBL / Temple Length (e.g., 52-16-140). DBL should correspond to the patient's nose bridge width for proper fit. A DBL that is too narrow causes the frame to sit too high and press on the nose; too wide causes it to slide down. DBL is different from the nose bridge width of the frame hardware, though related.
Question 5: Which frame adjustment is used to correct a frame that sits too high on the patient's nose?
- Increase the splay angle of the nose pads to widen the pad spread (Correct answer)
- Increase the pantoscopic tilt
- Decrease the face-form angle
- Tighten the temple bend
Correct answer: Increase the splay angle of the nose pads to widen the pad spread
Splaying the nose pads outward (increasing pad splay) lowers the frame by widening the support base, dropping the frame to the correct position.
If a frame sits too high on the patient's nose, the nose pads should be splayed outward (increasing the splay/spread angle) to widen the pad contact area. This effectively lowers the frame. Conversely, if the frame sits too low, the pads are moved closer together (decreased splay) or moved upward. For metal frames with adjustable nose pad arms, the pad height can be adjusted by bending the pad arm up or down. For plastic frames with a solid nose bridge, the frame can be adjusted by warming and reshaping the bridge. These adjustments are fundamental dispensing skills tested on the ABO exam.
Question 6: What is the 'effective diameter' of a lens and how is it used in lens ordering?
- The diameter of the smallest circle that will contain the entire lens shape; used to determine minimum blank size (Correct answer)
- The distance from the optical center to the farthest edge of the lens
- The average of the horizontal and vertical lens dimensions
- The diameter of the lens at its widest point
Correct answer: The diameter of the smallest circle that will contain the entire lens shape; used to determine minimum blank size
Effective diameter (ED) is the diameter of the smallest circle encompassing the full lens shape. Minimum blank size = ED + 2× decentration, ensuring the blank covers the entire finished lens.
Effective diameter (ED) is the diameter of the smallest circle that will completely contain the traced lens shape. It is critical for blank size selection in lens ordering. The formula for minimum blank size is: Minimum Blank = ED + 2 × Decentration + Safety Factor (usually 2 mm). Decentration is the distance the optical center must be moved from the geometric center of the blank to align with the patient's pupil. If the blank is too small, the lens cannot be cut to the correct shape with the optical center in the right position (the lens will be 'cut out'). Proper blank size calculation prevents waste and ensures correct optical center placement.
ABO NOCE Basic Opticianry Spectacle Dispensing and Fitting Questions and Answers 2
What is the pantoscopic tilt and what is the standard recommended angle?