In optics an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length.[1] This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's focal length (d = f1+f2). A simple example of an afocal optical system is an optical telescope imaging a star, the light entering the system is at infinity and the image it forms is at infinity (the light is collimated).[2] Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by