Minimum Blank Size (MBS) & Frame Geometry: Why Round 46mm Frames Beat 56mm Rectangles

In ophthalmic dispensing under the standard Boxed System ($A, B, DBL$), the minimum diameter of raw lens blank required to cut the prescription is calculated by:

$$MBS = ED + 2(d) + 2\ \text{mm (edging allowance)}$$

Where:

• $ED$ (Effective Diameter): Twice the longest radius from the geometric center of the frame aperture to its farthest corner.
• $d$ (Decentration per eye): $d = \frac{\text{Frame PD} - \text{Patient PD}}{2} = \frac{(A + DBL) - \text{Patient PD}}{2}$.

In a rectangular frame, the sharp temporal and nasal corners extend far away from the lens center. In a $54\times 32\ \text{mm}$ rectangle, the $ED$ often exceeds $59\ \text{mm}$. Because minus lens thickness expands parabolically toward the furthest edges, the corners become gigantic wedges of plastic.

In a round or oval frame ($A = 46\ \text{mm}, B = 44\ \text{mm}$), the $ED$ is merely $47\ \text{mm}$. The lens perimeter is equidistant from the optical center, cutting off the thickest perimeter completely!

1. Match Frame PD to Anatomical PD: Select a frame where $(A + DBL)$ matches your PD within $\pm 2\ \text{mm}$. Zero decentration ($d = 0$) means the optical center sits right in the middle, minimizing temporal edge flare.
2. Choose Chunkier Acetate Rims: A 4mm thick cellulose acetate rim physically conceals edge thickness, whereas ultra-thin wire or rimless titanium mounts leave edges totally exposed.
3. Minimize Vertex Distance: Fitting the frame as close to the eyes as possible ($10\text{--}11\ \text{mm}$) dramatically reduces the 'small eyes' minification effect.