Decentration calculator
Decentration is how far each lens's optical center is moved from the geometric center of the frame opening to line up with the pupil: (frame PD − patient PD) ÷ 2, where frame PD = A + DBL.
Calculator
Decentration and minimum blank size
Enter the frame's boxing measurements and the patient's PD.
The formulas
- Frame PD = A + DBL
- Decentration per lens = (frame PD − patient PD) ÷ 2, or (frame PD ÷ 2) − monocular PD
- Minimum blank size = ED + 2 × decentration + 2 mm. Some texts add about 2 mm for edge chips or defects (Brooks and Borish, pp. 82–83); it is a rule of thumb, not a fixed standard.
Worked example
Frame 54 □ 16, ED 57 mm, patient PD 63 mm:
- Frame PD = 54 + 16 = 70 mm
- Decentration = (70 − 63) ÷ 2 = 3.5 mm in per lens
- Minimum blank = 57 + 2 × 3.5 + 2 = 66 mm
Common mistakes
- Forgetting to halve: 70 − 63 = 7 mm is the total, not the per-lens decentration.
- Using A instead of ED for the blank size. ED is usually larger than A.
- Measuring ED as the longest diagonal. It is double the center-to-farthest-edge distance.
- Adding decentration only once. A round blank must grow 2 mm for every 1 mm of decentration.
- Using a binocular PD when the two monocular PDs differ. Work each eye separately.
A decentration error induces prism; check it with the Prentice's rule calculator.
Frequently asked questions
How do you calculate lens decentration?
Per-lens decentration = (frame PD − patient PD) ÷ 2, where frame PD = A + DBL. With A 54, DBL 16 and a 63 mm PD, the frame PD is 70 mm and each lens is decentered 3.5 mm in.
What is frame PD?
Frame PD is the distance between the geometric centers of the two lens openings, found as A + DBL in the boxing system. It is also called the distance between centers (DBC).
Which way do you decenter the lenses?
Usually in, toward the nose, because most patient PDs are smaller than the frame PD. If the patient PD is larger than the frame PD, the optical centers are decentered out.
What is the minimum blank size formula?
Minimum blank size = ED + 2 × per-lens decentration + 2 mm. With ED 57 mm and 3.5 mm decentration, the minimum blank is 57 + 7 + 2 = 66 mm. Some texts add about 2 mm; it is a rule of thumb, not a fixed standard.
What is ED (effective diameter)?
ED is double the distance from the middle of the lens opening to the point of the eyewire farthest from it. It is not the longest diagonal, and it is never smaller than the A measurement.
Why add 2 mm to the minimum blank size?
Some texts add about 2 mm as an allowance for edge chips or defects (Brooks and Borish, System for Ophthalmic Dispensing, pp. 82–83). It is a rule of thumb, not a fixed standard.
How do you calculate decentration with monocular PDs?
Work each eye separately: decentration = (frame PD ÷ 2) − that eye’s monocular PD. With A 56, DBL 19, ED 58 and a 32 mm right PD, the decentration is 37.5 − 32 = 5.5 mm and the right minimum blank is 58 + 11 + 2 = 71 mm.
What happens if a lens is not decentered correctly?
The optical center does not line up with the pupil, so the lens induces unwanted prism by Prentice's rule (prism = cm × D). The stronger the lens, the more prism a small centration error causes.
More free optician tools
- Plus/minus cylinder transposition calculator
- Vertex distance calculator
- Prentice's rule prism calculator
- Lensometer simulator
Studying for certification? See the ABO certification guide.
References
- Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, p. 19 (boxing system, frame PD / DBC) and pp. 82–83 (minimum blank size, Example 5-5).
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