Lens (optics) in the accompanying diagrams), while "negative" "R" means that rays reaching the surface have already passed the center of curvature. Consequently, for external lens surfaces as diagrammed above, and indicate "convex" surfaces (used to converge light in a positive lens), while and indicate "concave" surfaces. The reciprocal of the radius of curvature is called the curvature. A flat surface has zero curvature, and its radius of curvature is infinity. If "d" is small compared to "R" and "R", then the "thin lens" approximation can be made. For a lens in air, "f" is then given by As mentioned above, a
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