ABO NOCE Basic Opticianry Frame Measurements and the Boxing System — Questions and Answers
Question 1: In the boxing system of frame measurement, what does the 'A' measurement represent?
- The vertical height of the lens opening
- The horizontal width of the lens opening (eye size) (Correct answer)
- The distance between the two lens openings
- The effective diameter of the lens
Correct answer: The horizontal width of the lens opening (eye size)
In the boxing system, the 'A' measurement (eye size or lens width) is the horizontal width of the smallest rectangle that can fully contain the lens shape. It is one of the three key frame measurements (A, B, DBL) used to specify frame dimensions and calculate optical centration.
Question 2: The DBL (Distance Between Lenses) in the boxing system is measured from:
- The geometric center of one lens to the geometric center of the other
- The nasal (innermost) edge of one lens opening to the nasal edge of the other (Correct answer)
- The optical center of one lens to the optical center of the other
- The temporal edge of one lens to the nasal edge of the other
Correct answer: The nasal (innermost) edge of one lens opening to the nasal edge of the other
DBL is measured horizontally from the innermost (nasal) edge of one lens opening to the innermost edge of the other — it is the minimum gap between the two lenses. This measurement directly represents the bridge span required and is used in frame PD calculations.
Question 3: Using the boxing system, the frame geometric center distance (frame PD) is calculated as:
- A + DBL
- 2A + DBL (Correct answer)
- A + 2(DBL)
- A × DBL
Correct answer: 2A + DBL
Frame geometric center distance (GCD) = A + DBL + A = 2A + DBL. This is the distance between the boxing centers (geometric centers) of the two lens openings. It is compared to the patient's binocular PD to determine how much decentration is needed.
Question 4: What is the 'effective diameter' (ED) of a lens frame, and why does it matter?
- The A × B measurement product; used to determine total lens area
- The diagonal of the boxing rectangle; used to verify the lens fits the tracer
- The diameter of the smallest circle that fully contains the lens shape; used to determine minimum blank size (Correct answer)
- The distance from the boxing center to the optical center; used to calculate induced prism
Correct answer: The diameter of the smallest circle that fully contains the lens shape; used to determine minimum blank size
Effective diameter (ED) is the diameter of the smallest circle that can completely enclose the lens shape. When ordering lenses, the uncut blank must have a diameter greater than the ED plus twice the decentration amount, ensuring enough lens material to cut the correct optical center position.
Question 5: A patient's monocular PDs are 32 mm OD and 31 mm OS. The frame has an A measurement of 52 mm and a DBL of 18 mm. What is the decentration (in mm) required per eye?
- OD: 0 mm, OS: 0 mm — no decentration needed
- OD: 3 mm in, OS: 4 mm in (Correct answer)
- OD: 4 mm in, OS: 5 mm in
- OD: 1 mm out, OS: 2 mm out
Correct answer: OD: 3 mm in, OS: 4 mm in
The boxing center per eye sits at (A/2) + (DBL/2) = 26 + 9 = 35 mm from the frame midline. Decentration OD = 35 − 32 = 3 mm in (nasally). Decentration OS = 35 − 31 = 4 mm in. The OC must be moved nasally to align with the pupil because the frame's GC is wider than the patient's PD.
Question 6: When a patient's interpupillary distance is SMALLER than the frame's geometric center distance, the optical centers must be decentered in which direction?
- Temporally (outward), to follow the pupils
- Nasally (inward), to align with the pupils (Correct answer)
- Superiorly, to reduce induced vertical prism
- No decentration is required because the power compensates automatically
Correct answer: Nasally (inward), to align with the pupils
If the patient's PD is smaller than the frame GCD, the pupils are closer together than the boxing centers. The optical centers must be shifted nasally (inward) to coincide with the pupils. This inward decentration also induces base-in prism, which the optician must account for per Prentice's Rule.
ABO NOCE Basic Opticianry Frame Measurements and the Boxing System
In the boxing system of frame measurement, what does the 'A' measurement represent?