Lenses have a front and back. They are more properly referred to as the convex and concave. The outside or front side of the lens is convex. The concave is the inside of the lens, or backside. Historically, both sides of the lens are important and the curves that are ground on them have moved from front to back and vice versa. At first, lenses were any piece of glass that was thick in the middle and thin on the edge. Soon, however, lenses were ground to different powers. This was the first instance where the backside made a difference. All the correction plus or minus was essentially on the concave or backside that may or may not have been concave (convex/convex). The front side on these lenses was almost flat or one and a half dioptres convex. These lenses were ground to simple spherical curves.
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