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18% grey is used in the grey card as a
base color. Both grey and white surfaces
of the grey card have their own
function. Whatever you used in film or
digital photography, an 18% grey card
can control expose value and balance the
color so that it helps to revert the
color truly and perfectly by measuring
the exposure value, white/grey balance
value and color temperature.
A grey card had been invented based on the
optical principle and the characteristic of
photosensitive film. The camera exposure
value is in direct ratio with the logarithm
of characteristic curve. Characteristic
curve shows two important features of
photosensitive material (they are: contrast
factor and exposure latitude), and it is in
coordinate with the sense of human sight in
reflection to the colors in ray. When color
changes as geometric progression, its
density will change as arithmetic
progression. If we try to get a series of
density values like 1:2:3:4:5……, the
exposure value will be increased like
1:2:4:8:16…., always in multiple.
If we set the reflection ratio to 100% (as
maximum) and decrease progressively as
common ratio of 2, we’ll get the density
value of full-level diaphragm: 100, 50, 25,
12.5, 6.25, 3.125…… if the reflection ratio
decrease progressively as square root of 2,
we’ll get the density value of half-level
diaphragm: 100, 70.71, 50.35, 36.25, 17.68,
12.5, 8.84, 6.25, 4.42, 3.125…… Actually
those values are just different grey values
in different grades. We take it that the
standard exposure value is 17.68, then those
values beside it will be its exposure
latitude.
Look at those density values above, from
number 100 to 3.125, the ratio is 32:1. The
exposure latitude in this ratio is exactly
included in the photosensitive range among
in color negative, black/white negative,
color reversal film and CCD. We’ll see that
17.68 is just right in the middle of those
series of numbers. Thus, it will be
excellent and precise to take 17.68
(approximation of 18%) as a base value,
which controls the exposure value,
photographic density and color balance.
In manufacturing of photosensitive products,
18% grey had become a standard in theory
because of its special characteristic. 18%
grey as a foundation used in the
photographic sensitivity of the film, the
exposure value of exposure photometer, CCD
and CMOS sensors. |
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