Well, I will agree that color slide film typically has a 5 stop range. However, 18% was chosen as a reference standard long, long before there was any such thing as color film. Furthermore, simply by changing the films speed index you can move the midpoint up and down on the film. It is not engraved in gold, but was simply selected as a consistant reference.
Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ----- Original Message ----- From: "Herb Chong" <[EMAIL PROTECTED]> > Message text written by INTERNET:[EMAIL PROTECTED] > >Nope nope 18% is not in the center, unless you say white paper can not > relect mor than 72%. As you can see above 12% is the center of a 7 stop > range and 6% is the center of a 9 stop range. Actually, IIRC, Adams > selected 9% as zone V which would make 18% zone VI.< > > it's based on what will reproduce on decent slide film. 5 stop range where > 2 1/2 stops above average will be featureless white and 2 1/2 stops below > average will be featureless black. 18% is half way on that scale. table and > chart on page 25 of John Shaw's Nature Photography Field Guide. optical > density range on my Gretag Macbeth Color Checker is 90% reflectance for > pure white and 3% reflectance for pure black. total range is about 30x or > about 5 stops. > > Herb... >
