Strong Gravitational Lensing by Schwarzschild Black Holes
  The properties of the relativistic rings which show up in images of a source when a black hole lies between the source and observer are examined. The impact parameters are calculated, along with the distances of closest approach of the rays which form a relativistic ring, their angular sizes, and their "magnification" factors, which are much less than unity. 
G. S. Bisnovatyi-Kogan and O. Yu. Tsupko