Datasheet

14
c h a p t e r 1: DI GI TA L I M AG I NG B ASIC S , WOR K FLOW, A N D C A L I BR AT I ON ■
Figure 1.9
Spot meter readings are
taken from the camera
position.
The Mysterious “K” Factor
The mid-gray patch is calibrated to 18 percent reflectivity; this is supposed to match the
50 percent luminosity that light meters are calibrated to—but, in fact, it is off by roughly /
to ½ stop. Why this is so is unclear. Many reference “gray cards” are referred to as 18 percent
gray. In the printing industry, 18 percent reectivity is considered halfway between the white of
paper and the black of printing inks. However, actual scene luminance is a bit different than the
brightness of printing ink on paper. Light meters are calibrated to an ANSI standard of 12.3 per-
cent reflectivity, which is a closer match to 50 percent 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 percent to represent medium gray. This may come from Kodak who continues
to manufacture and market an 18 percent gray card. Kodak’s cards are produced using print-
ing ink on an offset litho press and 18 percent gray actually refers to a printing specification.
Presumably, this makes it easier to manufacture.
The bottom line is that the 18 percent Kodak gray card is a bit “lighter” than the 12.3 percent
reflectivity that light meters are calibrated to—and so, if you expose at the reading you get
from the Kodak card, you will be underexposing by about ½ f-stop. Ansel Adams was aware of
this and made reference to a mysterious “K” factor in his book Camera and Lens. His instructions
there had you opening up by ½ stop from the reading off the card. In general practice, I prefer
the exposure precision afforded by adjusting exposure in / stop increments—therefore, to
avoid overexposing, I only open / stop from readings taken off 18 percent gray cards.
The Mysterious “K” Factor
The mid-gray patch is calibrated to 18 percent reflectivity; this is supposed to match the
50 percent luminosity that light meters are calibrated to—but, in fact, it is off by roughly
/
to ½ stop. Why this is so is unclear. Many reference “gray cards” are referred to as 18 percent
gray. In the printing industry, 18 percent reectivity is considered halfway between the white of
paper and the black of printing inks. However, actual scene luminance is a bit different than the
brightness of printing ink on paper. Light meters are calibrated to an ANSI standard of 12.3 per
-
cent reflectivity, which is a closer match to 50 percent 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 percent to represent medium gray. This may come from Kodak who continues
to manufacture and market an 18 percent gray card. Kodak’s cards are produced using print
-
ing ink on an offset litho press and 18 percent gray actually refers to a printing specification.
Presumably, this makes it easier to manufacture.
The bottom line is that the 18 percent Kodak gray card is a bit “lighter” than the 12.3 percent
reflectivity that light meters are calibrated to—and so, if you expose at the reading you get
from the Kodak card, you will be underexposing by about ½ f-stop. Ansel Adams was aware of
this and made reference to a mysterious “K” factor in his book
Camera and Lens
. His instructions
there had you opening up by ½ stop from the reading off the card. In general practice, I prefer
the exposure precision afforded by adjusting exposure in
/ stop increments—therefore, to
avoid overexposing, I only open
/
stop from readings taken off 18 percent gray cards.
592120c01.indd 14 6/17/10 9:50:00 AM