In today's blog, I'll be briefly discussing the sometimes mysterious topic of White Balance, sometimes referred to as Color Balance. In short, White Balance is simply the adjustment of the color in a photograph to render neutral tones, such as white, grey and black, so that they appear… white, grey and black! Because photographs capture light reflected, scattered or transmitted from/through objects, the color of the illuminating light source can impart a color cast to the image, altering the appearance of the scene. An extreme example would be a red light shining on a white surface; the surface will attain a red hue in the image. An obvious fix would be to change the color of the illuminating source, but this isn't always possible.
Because most light sources in the past were generated by heated filaments, or from the Sun, color is often described by a Color Temperature, and is given in degrees Kelvin. Warm objects emit a “black body” spectrum that can be characterized by a single temperature. Sunlight has a color temperature of about 5800K, while a tungsten filament has T~3000K, and a fluorescent light has T~5000K. An overcast day will have a color temperature of T~6500K, which means it appears blue; the warmer Sunlight is suppressed and the bluer light (due to Rayleigh Scattering in the atmosphere and to some extent in clouds) is enhanced. Higher color temperatures translate into “bluer” light.
[Note that this is easy to see on a dark night. The stars will appear to be different colors, from red to blue-white, because they have different effective temperatures!]
In the good old days of film, there were only a couple of ways to handle color balance. One was to use a film with different emulsions (e.g., Kodachrome, Ektachrome and Fujichrome). You could also use colored filters on the lens to compensate. Finally, colors could be adjusted during printing. With digital technology, color balance can be easily controlled in the camera or in post processing. So let's look at a few details.
Nearly all digital cameras have an automatic white balance setting, which is usually fairly accurate. However, there are many situations where the auto setting will not set the correct white balance. This is especially problematic when the scene is lit by multiple sources that have different color temperatures. In these situations, the camera can be switched to a specific white balance setting. These are typically, tungsten, fluorescent, bright daylight, overcast sky. Higher end cameras also allow the settings to be fine tuned. This is particularly handy for mixed lighting situations, but can be tricky.
When should you worry about white Balance? The short answer is “always!” But a better answer really depends on the situation. I've gotten in the habit of shooting almost all images in RAW. This mode provides the most flexibility for most photographic situations, even if I'm just trying get some snapshots for Facebook. This is because RAW format stores the native intensity information from the sensor with minimal processing, and with 14 to 16 bit precision as opposed to the 8-bit (256 individual brightness levels) typical of JPEG images (see this discussion about Color Depth). This added dynamic range enables very precise control over the final image in post-processing. And for this particular application, it means that you can control the final white balance in detail. This also makes it easy to apply different color corrections to different parts of an image.
However, there is a price to be paid in the form of much larger files, more computing power for post-processing, and additional steps in your workflow (images must be converted to TIFF or JPEG to be usable in image editing programs. So in some situations, like large panoramas, snapshots or some timelapse situations, using a standard white balance setting and JPEG compression will be fine. My Canon t2i has a cool mode that will store both RAW and JPEG versions of an image. This makes it easy to figure out when you can and cannot get away with using JPEG. As I alluded to, I like to use RAW, and especially in situations with high dynamic range with lots of bright areas and shadows, and when there are many different light sources.
Most descent photography applications like Lightroom, Aperture and Photoshop (with the RAW plugin) have handy white balance tools. You simply find a location in the image that should be white, click on it, and the software adjusts the white balance. I use this quite a bit, and have found that it works pretty well. But sometimes, there isn't anything white in the scene. So, I recently picked up a set of Digital Grey Kards. This is a set of black, white and grey plastic cards that you use to help either set a Custom White Balance setting for JPEG shots, or for adjusting white balance in RAW images during post-production. My set of cards from DGK was fairly inexpensive, and they are small making them easy to carry and use. I don't use them often, but for JPEG timelapse, they have helped in a few cases.
Finally, even though white balance is usually used to correct images for color cast to make them look more “natural”, white balance settings can also be used creatively to add a color cast to an image. Let's say you want to warm up an image to make it look a bit more like it was taken in the “Golden Hour”, you can change the white balance to enhance the warmer tones.
Well, that's it for today.