Which set of numbers should be used when reading the base curve of a lens with a lens clock?

The base curve of a lens refers to the curvature of the front surface of the lens, and this curvature is typically measured in diopters, which can indicate whether the lens is convex (plus) or concave (minus). When using a lens clock, the reading is taken on a specific scale that represents these curvatures.

The correct answer, which involves using the black or plus numbers, is accurate because the black numbers on most lens clocks correspond to the positive diopter measurements typically associated with convex lenses. These measurements indicate that the lens has a base curve that is convex in shape, which means it is thicker at the center than at the edges, suitable for correcting hyperopia (farsightedness) or for use in sunglass lenses and certain types of prescription eyeglasses.

Using the black or plus numbers allows for the proper identification and categorization of lenses designed to provide support for certain visual deficiencies, which is crucial in optical settings when making prescriptions or fitting lenses into frames.