Lens Diameter vs ED (Effective Diameter)
Yamout Optical Center • Learn ED, Decentration, MBS, and why Patient PD never changes.
YOC • Optical Training
Inputs
Manual training rule (when no tracer): ED = max(A, B) + 2 mm (ONLY +2).
Frame
Order
For minus lenses: larger diameter often means thicker edges.
OK
Estimated ED
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Frame PD (A + DBL)
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Decentration (per eye)
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MBS (ED + 2×Dec)
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Ordered Diameter
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Safety Margin (Dia − MBS)
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Recommendation: —
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Why Decentration is needed (and why PD never changes) Staff Training
Key rule
Patient PD never changes. PD is an anatomical measurement (distance between pupils). We never add decentration to PD.
What decentration really means
Decentration (per eye) is how much each lens optical center must be shifted to align with the patient’s pupils.
It depends on the difference between Frame PD and Patient PD.
Why diameter does not replace decentration
A larger lens diameter (like 65 mm) gives you more material to cut and to shift the lens, but it does not center the optics automatically.
You still must decenter so the optical center matches the pupil.
Example
If Patient PD = 64 and Frame PD = 70, the difference is 6 mm.
Each lens shifts inward by 3 mm (per eye). The final optical distance between the two lens centers becomes 64 mm (correct PD).
Why decentration increases blank size
When you shift the lens, you need extra lens material on one side. That’s why we calculate:
MBS = ED + (2 × Decentration)
MBS = Minimum Blank Size (minimum lens diameter required for safe cutting after centering).
Training logic used:
ED = max(A,B)+2 • Decentration = |FramePD − PatientPD| / 2 • MBS = ED + 2×Dec.
Visual
Ordered Lens Diameter
MBS (Minimum Blank Size)
What is MBS?
MBS (Minimum Blank Size) is the minimum lens diameter needed to cut the lens safely after centering.
If the ordered blank is smaller than MBS, the lens may not cut properly.
Where do I get ED?
Best source is a frame tracer or manufacturer data.
If not available, this training tool estimates ED using max(A,B)+2 mm.
