Virbhadra–Ellis lens equation

The Virbhadra–Ellis lens equation relates angular positions of the unlensed source ( β ) , the image ( θ ) , the Einstein bending angle of light ( α ^ ) , and the angular diameter lens-source ( D d s ) and observer-source ( D s ) distances.

This lens equation is mainly useful for studying gravitational lensing in a strong gravitational field (see here for more information.)