The boxing system draws a rectangle around each lens shape. A is its width, B its height, and DBL the gap between the two boxes at the bridge. ED doubles the longest line from the box middle to the edge. Frame PD equals A plus DBL; half the difference from the patient’s PD is each lens’s decentration.
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Why a standard system
Labs and dispensers need to describe a frame the same way. The boxing system does this by enclosing each lens shape in a rectangle whose sides just touch the shape, then measuring the box.
A, B, ED and DBL
- A: the box width, horizontal, across the widest part of the lens shape.
- B: the box height, vertical.
- Geometric center: the middle of the box.
- ED (effective diameter): double the longest line from the box middle out to the shape’s edge. It is not simply the longest diagonal.
- DBL: the distance between lenses, the shortest gap between the two boxes. Also called the bridge size.
Frame PD
Center to center, the two boxes give the frame PD (also called the geometric center distance). It equals A + DBL.
Decentration
When the patient’s PD is less than the frame PD, each optical center moves in by half the difference: (frame PD − patient PD) ÷ 2. With monocular PDs, compare each eye with half the frame PD. The decentration calculator does this, and minimum blank size shows how ED and decentration set the blank. See also how to measure PD.
Example: 54-18 frame, PD 64. Frame PD 54 + 18 = 72; decentration (72 − 64) ÷ 2 = 4 mm in per lens.
Reading the markings
Frames are usually marked with eye size and bridge, such as 52-16. The first number is A and the second is DBL, so frame PD can be found straight from the marking.
Practice set
Original problems.
Frame 50-20. What is the frame PD?
Show answer
50 + 20 = 70 mm.
Same frame, patient PD 62. Decentration per lens?
Show answer
(70 − 62) ÷ 2 = 4 mm in each lens.
The farthest edge point is 29 mm from the geometric center. What is the ED?
Show answer
2 × 29 = 58 mm.
A frame is marked 52-16. Which number is the DBL?
Show answer
16 mm; 52 is the A measurement.
Frame 52-16, monocular PDs R 31, L 33. Decentration for each eye?
Show answer
Half the frame PD is 68 ÷ 2 = 34. R 34 − 31 = 3 mm in; L 34 − 33 = 1 mm in.
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Standard text used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 2 (boxing system, ED, DBL, frame PD). Eyecare Business and 20/20 Magazine pieces are linked under sources.
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Frequently asked questions
What do the numbers on a frame temple mean?
The usual pair is the eye size (the A measurement) and the bridge (DBL), both in millimeters, so 52-16 means A 52 mm and DBL 16 mm.
Sources: Eyecare Business (February 2010) — Last Word (boxing system) (opens in a new tab) ; Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 2 Frame Measurements, pp. 17–19 (opens in a new tab)
What is the A measurement of a frame?
The horizontal width of the box that encloses one lens shape, from its farthest nasal point to its farthest temporal point.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 2 Frame Measurements, pp. 17–19 (opens in a new tab) ; Eyecare Business (March 2004) — Fix and Fit (frame-related measurements) (opens in a new tab)
Is DBL the same as bridge size?
Yes. DBL, the distance between lenses, is the narrowest gap between the two lens openings, and it is what “bridge size” means.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 2 Frame Measurements, pp. 17–19 (opens in a new tab) ; Eyecare Business (February 2010) — Last Word (boxing system) (opens in a new tab)
Official sources
- Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 2 Frame Measurements, pp. 17–19 (opens in a new tab)
- Eyecare Business (March 2004) — Fix and Fit (frame-related measurements) (opens in a new tab)
- Eyecare Business (February 2010) — Last Word (boxing system) (opens in a new tab)
- 20/20 Magazine — Thick or Thin: What Matters Most When it Comes to Lens Thickness (opens in a new tab)
Last reviewed . Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.