Paul,
Thankyou for your kind words! And for the link to the video ..... LOL - what a character! Very helpful illustration, and fun to watch as well! I'm still trying to put it all together .... so - does this mean that the focal length determines the aperture width? Can that be varied? If so, is that done to achieve certain effects? Please bear with me! I have only just begun to understand aperture widths and shutter speeds and 'metering' ... and still make a mess of it all very frequently (my 'trashcan' becomes full of discarded images!).