Lens (optics) concave surfaces is "biconcave" (or just "concave"). If one of the surfaces is flat, the lens is "plano-convex" or "plano-concave" depending on the curvature of the other surface. A lens with one convex and one concave side is "convex-concave" or "meniscus". It is this type of lens that is most commonly used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a "focus") behind the lens. In this case, the lens is called a "positive" or "converging" lens. The distance from the lens to the spot
Parallel projection has the following properties. Orthographic projection is derived from the principles of descriptive geometry, and is a type of parallel projection where the projection rays are perpendicular to the projection plane. It is the projection type of choice for working drawings. The term "orthographic" is sometimes reserved specifically for depictions of objects where the principal axes or planes of the object are parallel with the projection plane (or the paper on which the orthographic or parallel projection is drawn). However, the term multiview projection is also used. In "multiview projections", up to six pictures of an object are produced, with