"Anand DHUPKAR" <[EMAIL PROTECTED]> wrote:
>I think I missed some e-mails, however, there was some discussion on 13%
>grey or 18% grey. Would someone explain that part to me a bit ?
>More importantly, is there any book which would explain the zone system ?
There was a long thread on this subject a while back. To sum up:
ANSI PH3.49-1971, the U.S. standard for reflectance ligh meters (there is *no*
international standard, by the way!), specifies about 12.5%, not the 18% of
commercially available grey cards. Kodak says, when using an 18% grey card, "For
subjects of normal reflectance increase the indicated exposure by 1/2 stop."
Camera and light meter manufacturers, however, are free to calibrate their
meters any way they want so it's best to learn you *your* equipment performs
under real life conditions.
Some links:
http://www.richardhess.com/photo/18no.htm
This page shows the calculations involved. The math is basic algebra (but
it does involve some rather arcane units of measurement like footlamberts!)
http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000gMS
"Danny Cirillo of Mamiya America lists the reflectivity constant for Sekonic
meters as 12.5 and Minolta as 14. The Sekonic owner's manual list 12.5"
A book:
Ctein, "Post-Exposure: Advanced Techniques for the Photographic Printer."
(See "Metering and the Myth of 18% Gray" on page 29.)
--
Mark Roberts
www.robertstech.com
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