Decentration per lens is (A + DBL − PD) ÷ 2. Minimum blank size is ED + 2 × decentration + 2 mm, where the 2 mm is a common allowance for edging, not a formal standard. Example: A 52, DBL 18, PD 62 and ED 56 give 4 mm decentration per lens and a 66 mm minimum blank.
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The one-line formula is on ABO exam formulas. This guide covers monocular PDs, outward decentration and why blank size matters. Ophthalmic Products and Dispensing are 20% each of the ABO outline (test specifications, checked Nov 2, 2026).
Terms: A, B, ED, DBL, frame PD
- A: horizontal width of the lens opening (boxing system).
- B: vertical height of the lens opening.
- ED: effective diameter, twice the longest radius from the box center to the edge.
- DBL: distance between lenses (the bridge).
- Frame PD: A + DBL, the distance between the two box centers.
Decentration per lens
Binocular PD: (A + DBL − PD) ÷ 2. A positive result means the optical center moves in (nasal). Monocular PDs: (A + DBL) ÷ 2 − monocular PD for each eye separately.
The MBS formula
MBS = ED + 2 × decentration + 2 mm. The 2 mm covers edging and chipping. It’s a common rule of thumb, not a published standard, and some texts and labs use a different allowance.
Check your hand answers with the decentration calculator. It uses these formulas and works with binocular or monocular PDs.
Worked examples
Binocular PD
A 52, DBL 18, PD 62, ED 56. Frame PD = 70. Decentration = (70 − 62) ÷ 2 = 4 mm in each lens. MBS = 56 + 8 + 2 = 66 mm.
Monocular PDs
A 50, DBL 20, PDs R 31 / L 33, ED 54. Half the frame PD = 35. Right: 35 − 31 = 4 mm in, MBS 54 + 8 + 2 = 64 mm. Left: 35 − 33 = 2 mm in, MBS 54 + 4 + 2 = 60 mm. Order for the larger eye if one blank size serves both.
Why it matters
Blanks come in set sizes. A large MBS can rule out a lens design or material, and a bigger blank on a high-minus lens usually means thicker edges. Compare materials on the lens materials comparison and our lens materials guide. If the optical center ends up in the wrong place, Prentice’s rule tells you how much prism that creates.
Practice problems
Original problems. Use the 2 mm allowance.
A 54, DBL 16, PD 60, ED 57
Show answer
Frame PD 70. Dec = (70 − 60) ÷ 2 = 5 mm in. MBS = 57 + 10 + 2 = 69 mm
A 48, DBL 20, PD 64, ED 52
Show answer
Frame PD 68. Dec = (68 − 64) ÷ 2 = 2 mm in. MBS = 52 + 4 + 2 = 58 mm
A 56, DBL 17, PD 70, ED 60
Show answer
Frame PD 73. Dec = (73 − 70) ÷ 2 = 1.5 mm in. MBS = 60 + 3 + 2 = 65 mm
A 50, DBL 18, PD 70, ED 53
Show answer
Frame PD 68. Dec = (68 − 70) ÷ 2 = −1 mm, so 1 mm out. MBS = 53 + 2 + 2 = 57 mm
A 52, DBL 18, monocular PDs R 30 / L 34, ED 57
Show answer
Half frame PD 35. R dec 5 mm, L dec 1 mm. MBS R = 57 + 10 + 2 = 69 mm; L = 57 + 2 + 2 = 61 mm
Frequently asked questions
What happens if the lens blank is too small?
The edged lens can’t fill the frame with the optical center in the right place. You either get a gap at the edge, or the lab moves the optical center to make it fit, which puts unwanted prism in front of the eye.
Is effective diameter the same as the A measurement?
No. A is the horizontal width of the lens opening. ED is twice the longest distance from the geometric center to the edge, usually measured on a diagonal, so ED is normally larger than A.
Does decentering outward change the blank size formula?
No. Use the size of the decentration, ignoring direction. If PD is wider than the frame PD, the lens moves out, and 2 × that distance is still added to the ED.
Official sources
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.