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| p.1 #3 · How to nail the white balance accurately each and every single time in LR 4? |
The WB setting is a "baseline". For example if set to Daylight stuff in the sun will look "normal" (neutral) but things in the shade (where light comes from the blue sky) will seem cooler BY COMPARISON. Go inside and forget to change the camera to Tungsten and everything captured will be abnormal because the camera WB doesn't match the color temp of the lighting. The advantage of RAW is being able to shift the WB baseline in the file without any adverse effect as with a JPG capture.
In a situation like you describe where a scene doesn't have a neutral you can click on to create an R=G=B baseline if you use the same baseline for all other shots in the same lighting what you can do with RAW is first "click-correct" the baseline in a shot that does have neutral content then copy/paste the adjustment settings to the other files in the batch.
For example if you forget to switch from Daylight to Tungsten when moving indoors you find file with a non-clipping white highlight, click correct on it, then copy the WB metadata to the other files. That will work with any of the WB presets on the camera but not with AWB.
AWB tries to mimic the way our brains adjust perception to color temp automatically. It works by finding a highlight each shot after it is taken, assuming it is neutral, and shifting WB so it is R=G=B. It works OK when files contain large neutral content recorded under clipping, but in situations where the highlights are not actually white, such as a light pastel yellow or blue shirt, AWB will shift the overall WB in the opposite direction, recording the yellow shirt as white and making everything else in the photo look blue.
In terms of doing the batch correction described above the problem with AWB is the lack of any consistent WB BASELINE across all the files. With AWB each file is adjusted differently at capture and the balance in one shot may not work in all the others in a consistent, predictable way.
When possible it's best to photograph a gray card in a scene. The gray card is a process control baseline for color. Because the card reflects equal R=G=B in any color temp light (no all things do) it can be trusted to shift the color baseline of the RAW file to "technically" neutral when the card image is clicked in the file with the eye dropper. Then as described above you can batch correct all the other files without the card to make them neutral also.
Setting Custom WB off a gray card makes the "click to neutral" correction in the RAW file. It doesn't change the actual file values (no WB setting does) it simply puts the instructions for displaying it from an R=G=B on card baseline to the computer when displaying the RAW and also makes the JPG preview neutral.
If a camera is set to Daylight and you shoot a gray card in Tungsten light the card will look orange from that WB baseline. If you set Custom WB on the card and then shoot it again, the second shot will look neutral. By toggling between the two its possible to judge by comparison from the R=G=B baseline what the color temp of the light is. That's very useful in situations like shooting under trees.
Under trees the light bouncing of the leaves give it a green cast. But you won't notice it on the subject or in the playback of a Daylight WB shot because your eyes will adapt and see the light as "normal" (neutral WB). You can't trust your eyes to judge overall color balance because they adapt. But if you were to shot the card with Daylight WB, then again after setting Custom WB, your brain will be able to see BY COMPARISON that the Daylight shot has a heavy green bias. Why? Your brain now trusts the Custom WB baseline of the camera more that what was seen a few minutes earlier by eye.
For that reason I always set Custom WB for portrait sessions where "normal" skintones are expected in the results and then have the subject hold a gray card and color chart as a visual reference when editing. Because the file is seen R=G=B straight out of camera clicking on the card does not shift the color - its already neutral, as is any neutral clothing. But "technically" neutral isn't always the best "perceptual" rendering of skin. A gray card balanced shot will often seem cooler than normal because we have a preference for warm vs. cool lighting. But the dilemma is how to shift the skin warmer without changing the color balance in the white dress or gray sweater...
The solution provided in ACR and Photoshop is "selective" color editing. PS knows from the pixel RGB values what content is neutral and can keep them the same while shifting the color. In PS the color balance and hue/saturation functions change color but preserve the neutrals. It's not as intuitive in ACR.
In ACR there is a "camera profile" tab. That's where you can apply different "styles" to a file as in the camera. When Adobe adds a camera to ACR it copies what the styles to in the camera to similar menu picks on the "camera profile" tab.
If you take a subject and first click correct on a gray card, then go to the camera profile tab and select different styles you'll see the card stay exactly the same, neutral, as the colors shift around. Portrait style boosts saturation in reds (too much I think) when compared to the "Neutral" style. Landscape boosts saturation in green foliage and blue skies compared to Neutral. Faithful boosts saturation overall compared to Neutral.
There are also sliders on the camera profile screen. So in the case of a portrait where I want the skin a bit warmer I might first select the "Faithful" style from the menu then tweek the red slider a bit more which will increase the saturation in the skintone but not shift the dress and sweater like the temperature and tint sliders on main screen of ACR do.
Where you want to use the temp/tint sliders is to correct the overall WB baseline of a file that doesn't have desired WB out of camera on the screen. Forgot to change WB to Tungsten when moving indoors? The "temperature" slider with shift the color on the yellow/blue axis in the RAW file to get rid of the orange color cast. Shot under trees with Daylight and now have dull greenish, gray skin on your subject? Correct that OVERALL GLOBAL WB ERROR with an adjustment on the green/magenta Tint slider. Adding magenta will remove the green cast.
But as in person under the trees judging when the color is in fact neutral on even a perfectly calibrated monitor can be difficult. That's why it's better to have the gray card in the shot. Click the color to neutral first, them move the Temperature slider warmer / cooler and see what you like best perceptually BY COMPARISON WITH THE NEUTRAL BASELINE.
Do the same thing with the Tint slider. You'll find that green and magenta casts almost never look "normal". But next time you watch a TV show or movie note how the color balance changes scene-to-scene. Cinematographers frequently use a green bias in lighting to depict dull gloomy locations. It works like the light under trees to make skin tones look dull and gray, as if seen in a fog.
The takeaway? Use a gray card for the most consistent capture baseline, but realize that creatively "technical" R=G=B card balance is just the starting baseline for evaluating what works best emotionally to depict mood and environment. When you start using the Gray card baseline for comparison with warmer/cooler, greener/more magenta than "normal" you'll better understand how to control the creative dynamic of color balance.
Look on my web site for tutorials / examples of using a gray card for process control in the workflow and selective color adjustment examples: http://photo.nova.org