Most of us are aware of the importance of a Gray card
in reflected light metering.
Here is a little story behind it. I use
a gray card from Kodak which I bought from BnH here is the link http://bhpho.to/12kAXIg.
However there is a caveat before using it. Please read through.
However there is a caveat before using it. Please read through.
What is a 18% Gray card?
Many reference
“gray cards” are referred to as 18% gray. In the printing industry, 18%
reflectivity is considered halfway between the white of paper and the black of
printing inks. However, actual scene luminance is a bit different from the
brightness of printing ink on paper. Light meters are calibrated to an ANSI
(American National Standards Institute) standard of 12.3% reflectivity, which
is a closer match to 50% luminosity or L=50 (in Lab colorspace). For some unknown
reason, reference cards in use for
photography, if they are calibrated at all, choose 18% to represent
medium gray! This may come from Kodak who continues to manufacture and market
an 18% gray card. 18% gray actually refers to a printing specification (Kodak’s
cards are produced using printing ink on an offset litho press) and presumably
this makes it easier to manufacture. The bottom line is that the 18% Kodak gray
card is a bit “lighter” than the 12.3% reflectivity that light meters are
calibrated to and so, if you expose at the reading you get off the Kodak card,
you will be underexposing by about 1/2 f-stop. Ansel Adams was aware of this
and made reference to a mysterious “K” factor in his Camera and Lens book that
had you opening up by 1/2 stop from the reading off the card.
I prefer the incident light meter
reading or in reflected light meter an exposure precision afforded by adjusting
exposure in 1/3 stop increments; therefore, to avoid overexposing, I only open
1/3 stop from readings taken off 18% gray cards.
Try it. You will be amazed to see the results.
Try it. You will be amazed to see the results.
2 comments:
are you affiliated to b&H / Flipkart etc? do you really go through the books that you recommend?
do you make any commission on recommending these books. if so why not make a disclosure as most others do
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