Two frames can share the same A dimension while one is round and the other extends far into a diagonal corner. Because the uncut blank must cover the entire traced shape after decentration, horizontal width alone can underestimate what is needed.
01 · FARTHEST POINT
ED includes the point farthest from the geometric center
Effective Diameter is twice the maximum radius from the boxing center to the farthest point of the lens outline. When that farthest point is diagonal rather than horizontal, ED becomes more informative than A for blank coverage.
It is best understood as a geometric coverage value, not simply as the diameter of the finished lens.
02 · DECENTRATION
Decentration moves the required shape away from the blank center
When the wearer’s optical-center location differs from the frame’s geometric center, the shape must be positioned off center on the blank. A common quick estimate adds twice the horizontal decentration to ED.
That shortcut is not universal. Vertical decentration, prism, unusual shape, wrap and processing margin can make the full trace and actual blocking position more accurate.
03 · THICKNESS
Blank diameter can also influence finished thickness and weight
For plus lenses, a larger required diameter can increase center thickness depending on design and minimum-edge constraints. In minus lenses, large shapes and decentration can increase edge thickness and weight.
Using an unnecessarily large blank is therefore not always an optical or cosmetic advantage.
04 · TRACE
Digital tracing contains more geometry than one ED number
Modern tracers and edgers work with many points around the frame shape. ED remains useful for comparison and estimation, but the full trace is richer when the outline is complex.
05 · ORDER
Combine frame geometry with the wearer’s measured position
Blank sizing should consider ED, frame PD, monocular PD, fitting position, prism and the available diameter of the ordered lens design. In higher prescriptions, these choices directly affect finished thickness and appearance.
SOLUTION GUIDE
A practical check order
Look at ED and the full trace, not A alone.
Compare frame geometry with monocular PD.
Confirm the lens design and prescription are available in the needed blank range.
For higher powers, optimize frame choice and blank size together.
GLANOS CHECK
ED matters because it follows the farthest extension of the shape, which A alone may miss.
Final blank selection should still be confirmed with the full trace, decentration and the specific lens design.
CAUTION
A quick MBS formula is an estimate, not an absolute answer for every shape.
Irregular shapes, large vertical decentration, prism or high prescriptions deserve a full processing check.
GLANOS EYEWEAR
There is usually a reason behind eyewear discomfort.
effective diameter · ED · lens blank · minimum blank size · decentration · edging