this is a pretty good site for all sorts of information around the whole subject of photography., theres some very usefull information in there.,
http://www.morguefile.com/archive/classroom.php?MORGUEFILE=182b2iogkmn7t3ntbrlg0vvia4
http://www.morguefile.com/archive/classroom.php?MORGUEFILE=182b2iogkmn7t3ntbrlg0vvia4