21.1.13

18 percent gray card

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.

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.
I bought from BnH here is the link http://bhpho.to/12kAXIg

1 comment:

Anonymous said...

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