What does the ED value stand for in lens sizing?

What does the ED value stand for in lens sizing?

• A. Effective diameter (mm)
• B. External diameter (mm)
• C. Elliptical diameter (mm)
• D. Estimated diameter (mm)

Answer: (A)

In lens sizing, the ED value stands for the effective diameter—the functional width of the lens that actually sits on the cornea and interacts with the tear film. This measurement reflects the portion of the lens that determines fit, centration, and movement with blinking, not just the physical outer edge. The other terms aren’t used for this purpose: external diameter would imply a simple outer measurement, elliptical diameter suggests a noncircular shape, and estimated diameter implies a guess rather than a defined, functional size.

In lens sizing, the ED value stands for the effective diameter—the functional width of the lens that actually sits on the cornea and interacts with the tear film. This measurement reflects the portion of the lens that determines fit, centration, and movement with blinking, not just the physical outer edge. The other terms aren’t used for this purpose: external diameter would imply a simple outer measurement, elliptical diameter suggests a noncircular shape, and estimated diameter implies a guess rather than a defined, functional size.