By Carl Weese
Light, natural or artificial, indoors or out, is constantly variable. We notice the warm, reddish light of sunrise and sunset, but there are subtle variations all the time. We generally don’t notice the color of ambient light because our eye/mind perceptual system adapts, much as it adapts to changes in light intensity. For example, from outdoors on a dark day, the windows of a house will look yellow if there are tungsten lights inside. The observer standing outside is adapted to the color of daylight and sees the yellow color of the interior light, but if we enter the house we adapt almost immediately. From inside, the room light seems normal while the view out a window can look bright blue. Concentrate on what you see through the window, and the color will quickly look right again.
The yellow/amber light from a standard light bulb has a lower color temperature than daylight. Why is it referred to as temperature? A black body radiator (think of an iron bar or fireplace poker) gives off energy in the form of light when it is heated. First it glows ruby red. When it is heated to a temperature of 3200 Kelvin, it gives off light the same color as a typical studio photoflood, so the studio lamp is said to have a color temperature of 3200 Kelvin and that’s the calibration for a typical tungsten color film. Sunlight is the color given off by a black body radiator heated to 5000–5500K so that’s the color temperature daylight film is calibrated for. The color temperature of overcast sky is in the range of 5800 to 7000K, while open shade under a clear blue sky can hit 9000–12000K. That's why a "warming filter" is needed to keep pictures in shade from looking bright blue on slide film. At the other end, low wattage light bulbs in a living room lamp can be way down around 2000K and candlelight is lower still. These light sources, like the heated iron bar, produce a continuous spectrum of light. The lower temperatures make warmer, redder light, and the higher temperatures produce cooler, bluer, light.
Fluorescent tubes, sodium-vapor lamps, and other forms of high efficiency energy-saving lighting usually produce a discontinuous spectrum. This means that while the light from a “daylight” fluorescent looks somewhat similar to real daylight, it is deficient in specific wavelengths, mostly in the magenta area. Our eyes don’t adapt to this deficiency as well as to different temperatures of a continuous spectrum, which is why these light sources often look unpleasant and unnatural to us. Color film without correction records fluorescent light with a strong green cast and sodium-vapor with an even more obnoxious green/brown palette.
Photographic recording media—film, or digital sensors—do not have our ability to adapt to the color of ambient light. That’s why color transparency film has always been made in both daylight and tungsten versions. Color negative films can be adjusted with filtration when they are printed in order to compensate for different color temperature light sources with a high degree of success. Magenta filtration over the camera lens does a good job of taking out the green cast transparency film registers from magenta-deficient fluorescents, though the filter costs a stop or more in light transmission.
Sophisticated color meters like the Minolta unit introduced in the 1980s gave two readings of incident light—color temperature in degrees Kelvin, and a second reading on “the green/magenta axis” of a theoretical color wheel. After reading the light and consulting a filter chart, a combination of a warming or cooling filter with a green or magenta one would closely match the light to the film and so provide the most accurate color possible. Basically, with the right filters, the ambient light would be converted to the nominal 5000° of daylight film, or 3200° of tungsten film. This was a lot of work, and expensive too. The meter with additional flash-reading head cost over $1,000 and a workable set of fragile gel filters cost hundreds more. But it allowed for accurate color recording of subjects either in studio or out on location.
Now, with digital capture, the problem is still there, but we can deal with it in software. The sensor can’t change its sensitivity to match different sources of ambient light, but the software that interprets the sensor data either to make in-camera JPEGs or to develop RAW files on the computer can adapt. Not only is it much easier than the old days, but no filters are required and you don’t have to buy a color meter. With RAW files, you can get accurate color simply by shooting a reference exposure of a test target each time the light changes. With JPEG files you really need to set the white balance properly before shooting because a severe WB error will be beyond the adjustment possible to the 8-bit JPEG without major quality loss. Setting a custom white balance is essential here, and luckily it’s not too difficult, though some cameras facilitate this better than others. Digital cameras all have built-in software that attempts to get the white balance right, and this ability is steadily improving as new cameras are introduced, but no auto white balance system is right all the time.
When you bring a RAW file into ACR (the RAW file interpreter for Adobe Photoshop and Lightroom: other RAW processors will have equivalent tools but PS is what I use so it will serve for examples) the top section of the working window (a hint that this is the first thing you should do) is a panel with two sliders, called Temp and Tint. This should be familiar to anyone who spent years working with that Minolta color meter. Temp is our old friend color temperature, expressed in degrees Kelvin, while Tint is that green/magenta axis expressed in arbitrary units. If you have recorded a file of a test target like the WhiBal shown below, to achieve the most accurate color your camera system is capable of you just need to adjust those two sliders until you have a perfectly neutral rgb reading from the target. In fact, you don’t need to use the sliders but can instead use the eyedropper tool to click on the test target and ACR instantly sets the sliders. Then apply that white balance to all of the pictures you shot under those conditions. So, no meters, no filters, just remember to shoot a frame of the reference target each time your subject or ambient light conditions change.
One problem that even digital capture white balance can’t fix is "mixed lighting." If a room is illuminated with a combination of windows, tungsten light bulbs, and fluorescent fixtures, there is no single white balance that will correct for all of them at once, though sometimes you can hit a compromise that's quite pleasant. Here’s an example:
The large armory building housing a model railroad exhibit had lots of fluorescent lights up in the rafters, and lots of daylight pouring through high windows. The camera's auto white balance has tried to compensate—it looks a lot better than unfiltered slide film would—but the result still has a nasty green cast.
I've selected the test frame and a picture I'm interested in, then held down the shift key to turn the cursor into the eyedropper, and clicked on the light gray area of the WhiBal card. The White Balance sliders adjust for neutral balance on both files.
Next I can tweak some of the other settings to improve the tone of the "real picture" file. Because of the mixed light, the color was different around the room depending on how much daylight and how much fluorescent was present. Half a dozen test exposures of the WhiBal made it much easier to get realistic looking color from shots made in different parts of the space.
What’s so important about accurate color? Well, it depends on what you’re trying to do with your pictures. An obvious example where accuracy matters would be a commercial photograph that is meant to show potential customers what a subject looks like. Architecture, fashion, a product that features a “trademark color,” all might need the most accurate possible rendition. Reproductions of works of art would be another perfect example. Portraits generally benefit from getting the subject’s skin, hair, and eye color right. Documentation of anything from storm damage to nature specimens will be more useful the more correctly it renders subject colors. And nobody likes that telltale fluorescent green cast.
All photographic systems have color reproduction shortcomings. None are fully accurate even under ideal conditions. But with an exposure of a reference target and one quick maneuver at the beginning of RAW development you can be sure that you are giving your system its best shot at accurate color.
Should you always strive for technically accurate color in your pictures? That’s the topic of Part II, so stay tuned.
A limited number of places are still available for Carl Weese's next Platinum/Palladium Workshop at Daytona Beach, Florida, in April 2009. Click here for more information.
Featured Comment by Tim (excerpt): "From my own experience of product photography, I find camera sensors react differently to different colors. If I photograph three colored objects at the same time, for instance, red, green and purple. I find it almost impossible to match at least one of the colors regardless of white balance. It's like doing a puzzle, you'll get the red correct and the purple will be wrong. Any thoughts on this?"
Carl replies: This is exactly the point of setting a neutral white balance. All systems have deficiencies and color rendering errors, and they'll do better with some colors than with others. If you attempt to correct the color that's rendered the worst, it can make other colors go crazy, so it becomes a garden path leading right down a rabbit hole. With a neutral balance you get the closest possible "across the board" correction for a given system, but the deficiencies are still there.