Fisheye lenses Linear scaled (equidistant): r=f ·θ, where the distance of a point from the image center 'r' is dependent on the focal length of the optical system 'f', and the angle from the optical axis 'θ‘ (in radians). Practical for angle measurement e.g., star maps. PanoTools (Panorama Tools (also known as PanoTools) are a suite of programs and libraries originally written by the German physics and mathematics professor Helmut Dersch) uses this type. Orthographic: r = fsin(θ). …. Equal area (equisolid angle): r = 2fsin(θ / 2)... Stereographic (conform): r = 2ftan(θ / 2). etc.