cgardner
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gdanmitchell wrote:
And it isn't even that simple.
When it comes to white balancing, making white white is not always the best approach - though there are some situations in which in can be the target. Let me offer one example and may be point to some related ones.
Agree and expound on this in my tutorials on using a gray card and color process control in the photographic workflow.
Setting Custom WB off a gray card is simply a process control baseline; a way of putting the color into a "neutral corner" like a boxer after a knock-out so the as not to interfere with the referee. It is useful to shoot to a "by the numbers" baseline at capture in most, but not all situations because human color vision is so adaptive our eyes and brain are lousy color evaluation devices.
For example if shooting under a tree canopy daylight will take on a green bias. The photographer's brain knowing the subject is wearing a white shirt and what their skintone looked like under the clear sky will adapt his vision to make both look "normal" — as expected.
The RAW file values will represent the light color as seen by the camera in all cases, but how that color is initially displayed on the monitor and first seen by the photographer is controlled by the WB metadata tag in the file header. If the photographer, unaware of the green bias, were to shoot using Daylight WB on the camera the first impression of the file would be one which has the same green bias.
But then a quirk of human perception occurs. The photographer who didn't see the green bias in person will not see it on the monitor either because his brain still expects the white shirt and face to look normal. His brain will adapt his color perception to make it so.
I've seen this occur hundreds of times in outdoor photos taken under trees or next to foliage. A green bias renders skin, which is dominant in the red channel, with a dull flat look as if shot through a gray veil. That's because green added to red shifts it towards neutral gray. But at the same time the green bias makes the foliage hyper-saturated shifting its hue from a deep emerald green more towards lime green. Most providing C&C on an image like that will make comments that the lighting was too flat — lacked contrast because it fools the eyes and brain.
There's a reason for that on a deeper physiological level. The human eye has two types color sensing cone cells concentrated around the optic nerve (fovea) covering about 2% of the retina. They sense blue/yellow and green/magenta similar to how color is recorded in a RAW file and Lab file format. The remaining 98% of the retina is covered with rod cells which are only sensitive to a narrow band of light in the green region of the spectrum. The rods are also about 3000x more sensitive to light than the cones in the center — why we can see better out of the corners of our FOV at night. It is also why Bayer sensors have twice as many green sites as blue and red and why color spaces are larger in the green region — they all model the physiology of the eye and now the brain processes its signals.
Vision is a chemical process over the synapses in the cells of the eyes. Staring for any length of time at bright colors will fatigue color vision because the sensor cells in the eye quite literally "run out of gas". Due to the green sensitivity of the rods, looking bright green will tire out the eyes and affect color perception rapidly...
I managed offset printing plants for a living doing thousands of color OKs over the years and attended many training classes. An illustration similar to the one below was used in a class on color management I attended at the Graphic Arts Technical Foundation (GATF). GATF is often called on to arbitrate disputes between printers and customers. In the case below a customer had designed a cover in bright green with a neutral gray 50% tint of black grid pattern, and complained the printer had instead printed with a magenta bias...
http://super.nova.org/TP/GreenBoxGray.jpg
If you stare at it for awhile your perception will shift as the green maxes out the cells in the eyes and the perception of the 128,128,128 gray bars will change. You may even see darker spots at the four nodes the lines intersect. The same thing happens to some degree when you open and look a scenic photo where most of the content is green foliage.
The take-away from all that background information on human perception is that you can't trust your eyes to judge color accurately. If you want to see color from a objective baseline you need to adjust it "by the numbers". A gray card and Custom WB allow us do that in our photos.
Stop and consider that your current trusted baseline for "accurate" color is most likely your calibrated monitor. But way if you made a mistake while calibrating? How would you ever know? You have put blind faith in that hockey-puck as your process control baseline.
Let's say you did screw up your calibration process and the monitor has a red bias. You take a shot at noon with Daylight WB, open it on screen and guess what, it looks green, the opposite of the red bias on the monitor. So what do you do? Add red. That's how color goes off the tracks into the ditch very quickly. The color was correct out of camera, but was screwed up based on the how it appeared on screen.
As the green square example illustrates even when your monitor is perfectly calibrated the shift in your color perception will cause you to make incorrect color judgements if you sit and stare an image for a long time. That's one of the reasons a neutral gray walls around the editing space with D65 white point illumination is recommend, so you can look away from the monitor periodically and recalibrate your color perception to the known neutral baseline of the wall.
Custom WB works in much the same way IF after setting it you shoot the target again as an editing reference. For example if shooting under trees in green light I would start with my camera set to Daylight WB as a starting baseline, shoot the card, set custom WB, then shoot the card again...
http://super.nova.org/TP/GreenCast.jpg
Relative to Daylight WB the first shot has a green balance. Custom WB adds magenta — not to the RAW file but to the WB recipe for display — and the card is recorded as neutral in the second reference shot. That way every shot in the session, when first seen on the monitor is seen from that neutral baseline.
Toggling between the two frames on the camera my brain, intellectually knowing the second frame represents the baseline I should trust, can now see the bias in the lighting relative to Daylight WB I can't see in person because my eyes adapt.
Back on the computer I open the first test shot of the subject under the trees holding the gray card. Because I set Custom WB I know the card is neutral "by the numbers". If clicked with the color eyedropper is will not change. If it doesn't look neutral by eye what does it tell me? That my monitor isn't calibrated correctly. By using Custom WB you change your brain's trusted process control baseline from the hockey-puck to the camera that generates the color.
Is technically neutral WB the best color perceptually? That depends on the context of the environment seen in the photo. If the other clues in the photo say it was taken in the middle of the day then neutral WB will be perceptually correct. If there are long sideways shadows seen which tell the viewer the shot was taken near sunset the neutral WB will look unnaturally cool. If taken on a winter day where a cool bias conveys the impression it is cold outside (because skin turns blue when cold) the neutral baseline will look unnaturally warm. In other words "normal" color is a relative thing — relative to the other clues in the photo and what is expected.
A portrait taken at sunset is an interesting perceptual paradox. The overall impression of the lighting in person will be warmer than the camera's Daylight WB baseline and if shot with Daylight WB the sunset will appear in the photo as it was remembered in person. But put a portrait subject in the foreground and the shot taken with Daylight WB will render a in the sun face too warm to the point of looking jaundiced, or if lit by the skylight cooler than expected relative to how it was remembered in person. Why? Because in person color vision adapts to whatever content the brain is focused on and shifts to make that content "normal" per expectations. So regardless of the ambient color temp the brain will make faces look "normal". Not technically gray card neutral, but different than the camera will record it.
What is the solution to that dilemma? What I'd do face the subject east with their back to the setting sun and illuminate the front with flash, with custom WB set off the flash/diffusers. My diffusers warm the flash about 600K making it a close match to Daylight WB, so in effect I'd be shooting with Daylight WB.
When I open the test file with the subject holding the gray card the card will look "normal" — neutral as expected — the sunset in the background will look similar to my impression in person of its warmth, but the face will look unnaturally cold with the technically neutral WB. So in that reference test file I will adjust the color warmer from that neutral starting point until the face looks right — per the context of the scene and my in person impression of the natural lighting on it. Making the face warmer will also make the background warmer, but that will seem perfectly normal given the time of day.
Capturing from a neutral baseline is just the starting point not the end goal. The end goal is to make the color in the photo the same as my impression of the natural light. Knowing that shooting in natural light alone will not allow the camera to render it that way, and that in any lighting my brain will shift perception to normalized the face, I know from experience the best strategy to overcome the limitations of the camera is to "fake" the lighting on the face with flash to normalize it —first to the technically neutral baseline, then by eye on the monitor perceptually to the context of the environment.
The face in the photo will wind up warmer than I'd render it at noon or in the studio where more neutral WB is expected, and cooler that the camera set to Daylight WB would record it with just the ambient light. In that situation I wouldn't want to use Custom WB with just the ambient light because it would make the sunset beach look like high-noon color-wise. The face would look more normal in terms of WB in absolute terms, but that neutrality wouldn't match all the other clues in the environment surrounding the face.
There are other ways to handle a situation like that, such as setting WB to Flash then using a straw gell on the flash to warm it up a bit. But knowing how much gel to add is a judgement call that isn't easy to make on the beach with adapting color vision. That's why I prefer to shot with the face "technically" neutral at capture, the background perceptually accurate, then tweek the balance on the face to match the background on the monitor where I can just move the blue/yellow slider back and forth and find what looks "right" by comparison.
In a situation like shooting under trees I would not add flash for this reason...
http://super.nova.org/TP/GreenCastFlashMix.jpg
Setting custom WB off a gray card from the ambient light causes the camera to shift WB from Daylight to + magenta. Relative to that adjusted baseline any flash added will have a magenta bias due to the custom WB and a slight blue bias due to the fact flash is a bit cooler than daylight. The result in the photo will be a mixed lighting muddle where the shadows illuminated by the ambient fill will be neutral, but the flash lit highlights will have a magenta cast. On the computer when editing if color is adjusted to get rid of the magenta bias in the flash highlights the ambient shadows will shift from neutral to green.
The solutions to that color balance paradox, in order of preference are:
1) Don't shoot portraits under trees: An ounce of prevention is worth a pound of cure in PP and mixed lighting is an incurable problem. The same is true for any situation where the environment bounces a color cast onto the skin: brick walls, red barn, cars, etc.
2) Use reflectors, not flash: If a reflector is used for a portrait in the woods it will bounce the same green light the Custom WB has neutralized so there will be no mixed lighting. The reflectors can be used both as "key" sources to model 3D shape and as fill to lift the overall scene to record shadow detail. Keep the fill reflector in front of the case about even with the nose and the "key" reflector above the head where you would place a key light flash to mimic the direction of the key and fill components of the natural lighting.
3) Set Custom WB to ambient then gell the flash green to match: While technically feasible its not practical because its will be difficult to judge by eye from the playback when the balance of green in the flash is an exact match.
Making decisions like that require experience to understand the cause and effect and need to start with the goals for the shot.
When there is face in a photo the goal is to make it seem "normal" similar to seen by eye and there's a relatively narrow range of what will seem normal in in a photo because of the way the eye adapts in person.
For landscapes and scenics there is a wider latitude for what color balance looks normal. Generally speaking the clues to color balance come from the angle of the shadows. If the shadows lead the viewer to understand it is the middle of the day they will expect technically neutral WB. But if the shadows are long and sideways they will expect a warm glow in the lighting. Either way all you need to do is keep the camera set to Custom WB.
I never use AWB which changes WB shot-to-shot based on content. With Custom WB or a pre-set like Daylight outdoors or Tungsten / Fluorescent indoors I can correct my test shot with the reference gray card by eye to taste, then batch copy the settings to all the other RAW files. AWB will get the WB perceptually correct some of the time but doesn't allow for batch adjustment which is huge time saver.
The advantage of a gray card vs. filter-type WB tool is the ability to put the card in a reference shot with the subject holding it next to their face. If custom WB is set with an over-lens device it must be pointed at the light source, not the subject, and can't be used in the shot as editing reference.
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