An optical system includes a diffractive optical element having a diffraction grating provided, on a lens surface having curvature, in a concentric-circles shape rotationally-symmetrical with respect to an optical axis. A sign of the curvature of the lens surface having the diffraction grating provided...http://www.google.ca/patents/US20020015231?utm_source=gb-gplus-sharePatent US20020015231 - Optical system having a diffractive optical element, and optical apparatus
