ABO Ophthalmic Dispensing 5 — Questions and Answers
Question 1: A patient requires 2Δ base-down prism OD and 2Δ base-up prism OS. This is an example of:
- Yoked prism
- Compensating prism (Correct answer)
- Slab-off prism
- Fresnel prism
Correct answer: Compensating prism
Compensating prism uses opposite bases in each eye to correct a vertical phoria, as opposed to yoked prism where both eyes receive the same base direction.
Question 2: Slab-off (bicentric grinding) is most commonly used to correct:
- Horizontal phorias in single vision lenses
- Vertical imbalance at the near zone in high anisometropic prescriptions (Correct answer)
- Chromatic aberration in high-index lenses
- Progressive lens corridor swim
Correct answer: Vertical imbalance at the near zone in high anisometropic prescriptions
Slab-off creates a prism difference at the near zone to compensate for the vertical prismatic imbalance caused by anisometropia when reading through bifocals.
Question 3: The vertex distance significantly affects the effective power of lenses with power greater than approximately:
- ±1.00 D
- ±2.50 D
- ±4.00 D
- ±5.00 D (Correct answer)
Correct answer: ±5.00 D
Vertex distance compensation becomes clinically significant for powers of ±5.00 D or higher, where small changes in distance materially change the effective power at the eye.
Question 4: Which lens treatment absorbs UV radiation below 400 nm and is built into the lens material rather than applied as a coating?
- AR coating
- UV inhibitor (UV absorber) (Correct answer)
- Mirror coating
- Hard coat
Correct answer: UV inhibitor (UV absorber)
A UV inhibitor is incorporated into the lens monomer during manufacturing, providing UV400 protection as an inherent lens property, not a surface treatment.
Question 5: When dispensing a frame to a patient with a high nose, which bridge adjustment is most appropriate?
- Increase the splay angle of the nose pads (Correct answer)
- Decrease the splay angle of the nose pads
- Widen the DBL
- Increase pantoscopic tilt
Correct answer: Increase the splay angle of the nose pads
Increasing the splay (flare) angle of the nose pads spreads them wider to accommodate a higher nose bridge and raise the frame on the face.
Question 6: A Fresnel prism is most accurately described as:
- A permanently ground prism on the back surface of the lens
- A thin, lightweight press-on prism made of concentric prismatic rings (Correct answer)
- A prism used only in sunglasses
- A type of progressive lens design
Correct answer: A thin, lightweight press-on prism made of concentric prismatic rings
A Fresnel prism is a thin plastic press-on membrane with concentric prismatic grooves that approximates the effect of a full glass prism with minimal weight and thickness.
Question 7: The American National Standards Institute (ANSI Z80.1) tolerance for cylinder axis on a prescription of -1.50 D cylinder is:
- ±2°
- ±5° (Correct answer)
- ±7°
- ±10°
Correct answer: ±5°
ANSI Z80.1 specifies a cylinder axis tolerance of ±5° for cylinder powers between 0.25 D and 2.00 D.
ABO Ophthalmic Dispensing 5
A patient requires 2Δ base-down prism OD and 2Δ base-up prism OS.
This is an example of: