RE: WB, Gray Cards and Cloudy Days
RE: WB, Gray Cards and Cloudy Days
- Subject: RE: WB, Gray Cards and Cloudy Days
- From: Wayne Bretl <email@hidden>
- Date: Sun, 18 Aug 2013 15:12:08 -0500
The human visual system does many things in parallel processes, which may be confusing because we are used to thinking of the conscious result as one thing - either it is white balanced and neutral or not. Actually, the identification of white objects and the perception of their color are two different processes. The truth of this became clear to me after having a cataract operation on one eye. The operated eye has a UV-absorbing implant that approximates the spectral absorption of a youthful natural lens. (Clear implants also are commonly used - these absorb much less near UV, with the result that certain things, like gas flames, appear unusually bright, and daylight takes on a violet tinge. This has been reported by color scientists and others who have clear implants.)
The UV absorbing lens in one eye absorbs considerably less in the deep blue/violet than the 69 year old natural lens in my other eye.
The result (which I did not expect) is that white objects look white in either eye, but their color never matches - always warmer in the untreated eye. So, I conclude that there is not a perfect convergence to a certain neutral white color in the visual system - differences are still detectable even when adapted to the current illumination. (A persistent and easily discernible difference between white balances in two eyes has also been reported to me by a colleague who has one clear and one UV absorbing implant.)
How should one take this into account when adjusting white balance in photographs? Since there is no way to measure this internal white balance of your visual system for changes in illumination, it seems your only choice is to do what you have been doing, and adjust for the closest memory color, which will be cooler on cloudy days, but not as much as the result of keeping the camera white balance setting constant.
I find the same thing you do in my photography - a partial compensation for illumination color looks more natural to me than truly neutral.
Wayne B
-----Original Message-----
From: colorsync-users-bounces+waynebretl=email@hidden [mailto:colorsync-users-bounces+waynebretl=email@hidden] On Behalf Of Louis Dina
Sent: Sunday, August 18, 2013 7:33 AM
To: Colorsync Users List
Subject: WB, Gray Cards and Cloudy Days
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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