Strehl ratio and optimum focus of high-numerical-aperture beams Augustus J.E.M. Janssen, Sven van Haver, Joseph J.M. Braat and Peter Dirksen J. Eur. Opt. Soc.-Rapid Publ., 2 (2007) 07008 Published online: 23 February 2007 DOI: 10.2971/jeos.2007.07008