I was intrigued by a statement I made during a review of a sample exam session and this afternoon decided to find out what the new f# should be given that one wants to increase the light level at the image plane over that available at some starting f#. After 4 hours of pondering came up with this (whole increments would be quite easy ...): http://www.davidhazy.org/andpph/text-fnumber-aperture-adjustment.html I am sure there are easier ways to do this and if so a hint would be most welcome! Of course fractional or "decimal" f#s are not easy to set! Andy
