The action of a mass as a lens is partly comparable with a focussing optical lens. Depending on the constellation of the source, the lens, and the observer and depending on the form of the lens different patterns of lensed images arise.

If the source, the lense and the observer are situated on a straight line and if the lens is spherical then the image appears as a ring - the Einstein Ring (top).

For a non-spherical lens one typically gets the Einstein Cross (middle).

For more general lenses more complex images are formed, typically in the form of arcs and arclets (bottom).

One can directly calculate the mass distribution of the lens from the position and the distortion of the images.