The symmetry axis of a lens or a lens system.
The optical path length is proportional to the time light needs to travel between two points
F# with the largest possible DOF.
Larger F# reduces the DOF due to diffraction
orthographic lenses are a type of fisheye lenses
maintains planar illuminance
An orthographic / hemispherical fisheye, uses a parallel projection of a hemisphere onto a plane. The resulting image will obviously be circular. The widest view angle is 180 degrees. At +/-90 degrees extreme distortion will occur. Due to this heavy distortion it’s less often used than f-Theta / angular fisheye.