Understanding Minimum Blank Size
Minimum Blank Size (MBS) is the smallest diameter lens blank that will completely fill a frame opening after the lens has been decentered and edged. Ordering the correct blank size is essential for efficient lens fabrication. A blank that is too small means the edged lens will have a chip or missing edge, making it unusable. A blank that is too large wastes material and may add unnecessary thickness and cost.
Think of it like cutting a circular piece from a round sheet of material. If you need to cut off-center (because of decentration), you need a sheet large enough that the cut piece is still complete, even at the farthest edge.
The MBS Formula
The standard formula is:
MBS = ED + (2 x Decentration) + 2mm
Each component serves a specific purpose:
- Effective Diameter (ED) is the longest measurement across the lens shape from any angle, passing through the geometric center. Unlike the A measurement (horizontal width), ED captures the true maximum span of the lens shape, regardless of its orientation.
- Decentration is the horizontal shift needed to align the optical center with the patient's pupil. You multiply by 2 because the shift in one direction means you lose that same amount of coverage on the opposite side.
- 2mm safety margin provides a processing cushion for edging tolerances and minor measurement variations.
Step-by-Step Calculation
Consider a frame with these specifications:
- Effective Diameter: 54mm
- Frame PD (A + DBL): 72mm
- Patient PD: 64mm
First, calculate decentration: (72 - 64) / 2 = 4mm
Then apply the formula: MBS = 54 + (2 x 4) + 2 = 54 + 8 + 2 = 64mm
You would order a 65mm blank (since blanks come in standard sizes, round up to the next available diameter).
Effective Diameter Explained
The Effective Diameter is measured from the geometric center of the lens opening to the farthest point on the frame's lens groove, then doubled. This captures the worst-case scenario for blank coverage.
Most frame manufacturers provide the ED in their specifications. If it is not listed, you can measure it with a ruler or pupillometer by finding the longest possible line through the geometric center of the lens shape. In practice, the ED is often 2-6mm larger than the A measurement for standard frame shapes.
Why the Decentration Factor Is Doubled
This is a common source of confusion. When you shift the optical center nasally by 4mm, the lens blank effectively moves 4mm toward the nose. This means you gain 4mm of coverage on the nasal side but lose 4mm on the temporal side. The blank must compensate for this loss, so you add the decentration twice: once for the shift itself and once for the coverage gap it creates.
Vertical Decentration Considerations
The standard MBS formula accounts for horizontal decentration only. In some cases, you also need vertical decentration, such as when fitting progressive lenses with specific fitting heights or when compensating for uneven segment placement. If vertical decentration is significant, you can modify the formula:
MBS = ED + 2 x sqrt(Horizontal Dec² + Vertical Dec²) + 2mm
This uses the Pythagorean theorem to calculate total decentration when both horizontal and vertical shifts are involved. However, for most standard dispensing situations, horizontal decentration alone is sufficient.
Practical Considerations
Standard Blank Sizes
Lens blanks are manufactured in standard diameters, typically increasing in 5mm increments (55mm, 60mm, 65mm, 70mm, 75mm, 80mm). Always round your MBS up to the next available size. Ordering a blank that is exactly the MBS value leaves no room for error.
Impact on Lens Thickness
Larger blanks mean more material, which translates to thicker, heavier lenses. For minus prescriptions, the edge becomes thicker. For plus prescriptions, the center grows thicker. This is one reason why matching the frame PD to the patient PD is so important: smaller decentration means a smaller MBS, which means thinner, lighter lenses.
Key Takeaways
- MBS = ED + (2 x Decentration) + 2mm is the standard formula
- Effective Diameter measures the longest span across the lens shape, not just the horizontal width
- Decentration is doubled because shifting the optical center reduces coverage on the opposite side
- Always round up to the next standard blank diameter
- Larger MBS values lead to thicker, heavier lenses
- Choosing frames with PDs close to the patient's PD reduces decentration and MBS requirements