
 photographic lens apertures (Site not responding. Last check: 20071016) 
  Currently this adjustment is almost always calibrated in fstops or fnumbers, which, roughly speaking, are the diameter of the glass at the front of the lens divided by the diameter of the hole made by the iris diaphragm. 
  To double the amount of light passing through the lens, the area of the aperture must double, and for that the new diameter of the aperture must be the square root of 2 (1.414…) times the old diameter, because the area of a circle varies as the square of its radius. 
  A lens has its maximum possible resolution at its widest aperture, but in practice, as a rule of thumb a lens is sharpest about 2 whole stops below its widest aperture. 
 www.sizes.com /tools/photoaper.htm (458 words) 
