WB, Gray Cards and Cloudy Days
WB, Gray Cards and Cloudy Days
- Subject: WB, Gray Cards and Cloudy Days
- From: Louis Dina <email@hidden>
- Date: Sun, 18 Aug 2013 07:32:44 -0500
I'd like feedback on this topic, especially if there are any hard color
science and studies to back it up. I'm also interested in general opinions
and observations, even if not backed up by science.
If I use a spectrally neutral gray card on a heavily overcast day, and use
it to set WB in my Raw Converter, I usually find that my images end up
being much warmer than the scene I perceived. This phenomenon is
independent of the camera profile, raw converter, monitor calibration and
other links in the color chain, and while they all affect the result,
sometimes significantly, my interest is primarily in how best to use a gray
card to approach reality as we perceive it. What I do with the color
afterward for artistic purposes is a separate matter.
I know our visual systems do an internal Auto WB to correct for the color
temperature of the prevailing light. However, if I drive around on a very
heavily overcast, rainy day, the scene before my eyes still remains
decidedly cool and deficient in the longer, warmer wavelengths. By
contrast, a sunny day adds life a warmth to the same foliage. And at
sunset, there is a distinct warmth and glow. It is my belief that our
visual systems move the internal WB in the direction of neutralizing the
light, but don't take it all the way to perfectly neutral. So, a gray card
shot in "normal" daylight, heavy overcast and at sunset, will be perceived
differently. At noon under 'white light' it will appear neutral, on a rainy
day it will appear slightly bluish, and at sunset it will appear more
yellow-orange.
Back to my example. If I shoot a gray card under gloomy skies, and accept
that as the correct color temp for this image in my raw converter, I am
forcing the card to be perfectly neutral, when I believe our brains see it
as cool-gray, no longer spectrally neutral. This makes sense to me because
the trees look cool and less warm just driving around on a gloomy day. If
this is correct, and I think it is, accepting the gray card WB forces the
scene to be artificially warm and overly yellow. Same thing happens under
incandescent light, around a campfire, at sunset, but this time the scene
is forced to be artificially cool compared to what our brains really see.
I still like using the gray card, because it provides a stable, measurable,
neutral point of departure, a baseline, if you will. But, if ACR/LR gives
me an overcast reading of 6500K/+5 Tint with the gray card, I find the
resulting image to be overly warm. Lowering the temp by approximately 500K
seems to always give me a more realistic image, more in line with my
remembered perception of the scene. I know, this is perception, not
science, and I also understand choice of camera profile, converter, monitor
calibration, etc, all play a role in the final color, but the same
phenomenon is at play.
I'd be interested in hearing others' thoughts on this subject. I am
particularly interested in any in-depth studies on the subject that relate
to how the human visual system does its WB, and if it forces gray to be
totally neutral as the light source diverges farther and farther from
"normal" daylight.
Thanks,
Lou
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