Two methods of image formation in lens design. The first is perspective (F-TanTheta) projection also called Rectilinear or Orthoscopic. The second method is Equidistant (F-Theta) projection also called Equiangular. The Perspective method is most often used during the design of the lenses with small and middle field of view for example in the 40- 60 degree range. Equidistant projection is used in wide-angle lenses like a fisheye lens. It bends straight lines but provides more than 180 degrees of lens FOV. There are 3 less common methods are Stereographic, Equisolid and Orthographic projections.