Thin lens positive if the surface is concave, and negative if it is convex. This is an arbitrary sign convention; some authors choose different signs for the radii, which changes the equation for the focal length. Certain rays follow simple rules when passing through a thin lens, in the paraxial ray approximation: By tracing these rays, the relationship between the object distance "s" and the image distance "s′" can be shown to be which is known as the thin lens equation. In scalar wave optics a lens is a part which shifts the phase of the wave-front. Mathematically this can be understood
Parallel projection central or perspective projection, in which the rays pass through a fixed point called the "center" or "viewpoint", as this point is moved towards infinity. Put differently, a parallel projection corresponds to a perspective projection with an infinite focal length (the distance between the lens and the focal point in photography) or "zoom". In parallel projections, lines that are parallel in three-dimensional space remain parallel in the two-dimensional projected image. A perspective projection of an object is often considered more realistic than a parallel projection, since it more closely resembles human vision and photography. However, parallel projections are popular in