Understanding Minimum Blank Diameter (MBD)
Minimum Blank Diameter (MBD) is one of the most critical pre-edging verification steps in any optical dispensing lab. Before a lens blank is mounted on the edger, the lab technician must confirm that the uncut blank is large enough to cover the entire frame eyewire after the lens is decentered to match the patient’s pupillary distance (PD).
The fundamental MBD formula is: MBD = Effective Diameter (ED) + Total Decentration + 2 mm safety margin. The Effective Diameter is the longest diagonal measurement across the lens shape (from the geometric centre of the eyewire to the farthest edge, doubled). Total Decentration equals the absolute difference between the Frame PD (A size + DBL) and the patient’s monocular or binocular PD.
Step-by-Step Worked Example
Given a frame with A = 54 mm, DBL = 18 mm, ED = 56 mm, and a patient PD of 64 mm: Frame PD = 54 + 18 = 72 mm. Per-eye decentration = (72 − 64) / 2 = 4 mm inward. MBD = 56 + 4 + 2 = 62 mm. A standard 65 mm blank is sufficient; however, a 60 mm blank would fail.
Common Blank Sizes & Frame Compatibility Table
| Blank Diameter | Typical Use | Max ED + Dec |
|---|---|---|
| 55 mm | Small child frames, low dec | 53 mm |
| 60 mm | Standard adult, narrow frames | 58 mm |
| 65 mm | Most adult frames | 63 mm |
| 70 mm | Large aviator / sports wrap | 68 mm |
| 75 mm | Oversized fashion, high dec | 73 mm |
| 80 mm | Safety goggles, extreme wrap | 78 mm |
Frequently Asked Questions
What happens if the blank is too small?
The edger will cut into the unfinished edge, leaving a visible chip or flat spot on the temporal or nasal side of the lens. The job must be remade with a larger blank or a smaller frame selected.
Does MBD change for progressive lenses?
Yes. Progressives require additional consideration for fitting height and corridor length. The minimum fitting height plus the distance from the optical center to the far edge must be checked independently.
Why add 2 mm safety margin?
The 2 mm accounts for edger chuck grip, bevel placement variation, and minor lens-shape irregularities. Without it, borderline blanks frequently fail during cutting.
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