Hi, I am not sure we discussed this on the list sometime in the past so if we
did forgive me. Someone posed the following question to me:
"What is the RGB value for 18% gray (or 18% grey for that matter) in Photoshop?"
This simple question led to three somewhat different answers from three
instructors in the vicinity.
One simply said: 177
Another said:
RGB is linear scale. It all depends on how your camera is set up. If
it is linear from 0-256 - some cameras are set up this way - then its
18% of 256 or 46!
And yet another:
The only thing I have to add is that the sensors are linear (mostly) and
so is the associated RAW data. If you has access to such RAW data, then
Don's calculations are valid.
However, the data the user generally has for use in programs like
Photoshop (is invariably GAMMA CORRECTED for perceptual uniformity
during image editing... The gamma correction used will vary by the type
of color space used (2.2 for sRGB/A-RGB but 1.8 for Profoto RGB).
(Incidentally, the moment a RAW file is opened in PS, it is GAMMA
corrected).
So basically, one has to undo this GAMMA correction before assuming
linear data. After that, you take 18% of whatever your scale is (0-255
for 8 bit, 0-65535 for 16 bit etc)
P.S.: Incidentally, meters don't use 18% gray for calibration...I think
ANSI standards are more like 12%. But then, does anyone use hand-held
meters anymore in this digital world :-)
...... so, what to make of all this? Anyone on the list have something to
contribute???
;)
andy
