Today's post is an excerpt from the book Small Flash Photography, by Bill Hurter. It is available from Amazon.com and other fine retailers.
The Color of Light
The color of light affects the way colors are recorded in a scene and is, therefore, of concern to all professional photographers. The color of light is measured in degrees Kelvin, based on a system devised in the 1800s by British physicist William Kelvin. Kelvin heated a dense block of carbon and noted that, at different temperatures, it emitted a repeatable and measurable color of light. The particular color of light seen at a specific temperature is now called the color temperature. When the carbon, also known as a “black body radiator,” is hot enough and just begins to emit light, it is dull red. As more heat is applied, it glows yellow, and then white, and finally blue.
Electronic flashes also have specific color temperatures. For example, the color of the light emitted by a flash may be rated at 5500K when it is designed to imitate noon daylight. If the flash produces light that is 6000K, it will be on the cool (bluish) side. If it is rated at 4800K degrees, it is slightly warmer (more yellowish) than white light.
Jeff Kolodny combined at least three different light sources in the scene to make a beautiful bridal formal. The doorway was lit by dim daylight with a blue tinge, while the room light was a combination of much warmer light from the chandeliers and other room lights. Jeff also popped a camera-mounted flash at less intensity than the daylight to help warm up the color balance and add a sparkle to the bride’s eyes.
Custom White Balance
When combining ambient light sources with flash, a custom white-balance reading should be taken. You may have room-lamp brightnesses to contend with, as well. Your best bet in these situations is to shoot a few test frames with flash and ambient combined to see where your white balance is. Then perform a custom white balance procedure to balance all of the light sources harmoniously. Alternatively, shoot in RAW capture mode, which will allow you to fine-tune the color balance after capture. The following is a resource for determining color temperatures in Kelvin degrees in the most popular lighting situations. These readings correspond to your menu settings for white balance and are provided in order to give you an insight into the Kelvin settings of most DSLRs.
The natural shade of late afternoon is very soft but lacks sparkle. To give the light a little extra snap, the photographer, JB Sallee, fired an on-camera flash that was set to output at two stops less than the daylight. The effect is exactly what the photographer intended.