Hi I have two general questions regarding a lens and its numerical aperture: 1) From this picture http://en.wikipedia.org/wiki/File:Numerical_aperture_for_a_lens.svg (from the Wiki-article http://en.wikipedia.org/wiki/Numerical_aperture) it seems to me that there is a propotionality between the NA of a lens and its focal length, i.e. long focal length = small NA. However this doesn't always seem to be true when I look at optical elements in various catalogs? 2) I read in an old thread (https://www.physicsforums.com/showthread.php?t=304834) that only when the full aperture of a lens is used will the total NA be available. Is it possible to give a rough estimate of how much of the NA is in use? Best wishes, Niles.