The base curve of a lens is the surface curve that acts as the foundation or starting point for all subsequent curves. The factory-finished curve will be the basis curve for semi-finished lens blanks, which is usually situated on the front of the blank.
The base curve is defined as "the curve from which all other curves are measured" in optical theory. We may conceive of the base curve as the front surface of the lens in contemporary lens design. That curve is always plus (+) in current lens design and has a convex form. The matching curves on the rear of the lens that generate the real lens power (bend light) are negative (-) and concave in shape.
The Nominal Lens Formula, the preceding lesson, discussed this relationship.
A part of surfacing is matching a base curve to a prescription. Surfacing is the process of removing the plastic from a lens in order to generate the curves required to meet a prescription. These days, surfacing is done in a "lab" rather than a "shop," and it is no longer part of an optician's day-to-day employment.
Certain elements of vision, including distortion and magnification, are affected by the base curve of a lens, and users may perceive perceptual variations between lenses with various base curves. As a result, some doctors may write "match base curves" into a new prescription.