Delicate Balance: WB and Your Camera
White balance can have a dramatic effect on your photos and the effect can be good or bad depending on how accurate the white balance was for a given shot. Let's take a quick look at this important but often ignored aspect of digital photography.
What is white balance?
Different types of light cast a different overall color onto objects. Incandescent (standard) light bulbs produce a warm, reddish light while fluorescent light tends to produce a cooler green-blue color. The "color" of the light source is often referred to as the "color temperature" with red being on the warm end of the spectrum and blue being on the cool end of the spectrum. Our eyes easily adjust to this difference so that a white piece of paper looks white under any color light, but cameras are a bit different. If the "raw" image data is viewed without compensating for the color of the light source itself, a white piece of paper will look red under warm lighting and blue under cool lighting.
We compensate for light sources with different color temperatures in photographic images by balancing the red-blue shift. If a sheet of paper appears in the photo, for example, and we know the sheet of paper is white, we can apply a red-blue bias to force the paper to be white and in doing so, the remaining colors in the photo will fall into place nicely. White balance basically amounts to adding/subtracting red or blue in the image until the red, green, and blue channels are equalized for neutral (gray or white) objects.
The balancing act
At this point, it may sound simple to just pick a neutral object in the photo and just rebalance based on that. Some photo editing software offers a "click to balance" option where a dropper is used to click on a neutral object. My own Qimage software offers this ability in the batch filter, for example. By clicking the dropper in the "White Balance" section and then clicking on a gray/white object in the photo, the entire image is rebalanced to remove color casts caused by improper white balance. A white shirt with RGB 200,225,245 will have a strong aqua/blue cast indicating a white balance error. Clicking on the shirt to rebalance will bring the shirt to RGB 225,225,225, making the shirt look white instead of blue by increasing the red channel to match the green channel and decreasing the blue channel to match the green channel. Why is the green channel not altered and only the red/blue channels changed? Because the green channel is generally considered the luminance or "brightness" channel. While technically the green channel is certainly not strictly brightness, it doesn't tend to shift like red and blue due to differing color temperatures.
By now, you may be thinking that this whole balancing act is no big deal. Just set the camera to automatic white balance (AWB) and hope it gets it right. If not, just click on a neutral object later and rebalance it or use curves to adjust manually. While that will correct color casts caused by the white balance error and will restore neutrality, there are problems with readjusting WB after the fact. If you are shooting in JPEG capture mode, your photos are being developed inside the camera. That means the camera has already decided how to process color based on the information available when the shot was taken. This color processing takes white balance into account and creates colors as they would appear assuming WB is correct. If you shift WB later, you can no longer benefit from the camera's complex color processing that must be done based on a correct WB reading and as a result, there may be some unwanted (but usually subtle) color shifts such as reds looking too orange, blues looking purple (or vice versa) and so on. If you are shooting in raw capture mode, this is less of a concern because WB can generally be corrected in the raw developing software, allowing the photo to be re(color)processed based on the change in WB. With already-processed JPEG's, it is impossible to remove the original color assumptions that were made based on an inaccurate white balance.
Getting it right from the get-go
By far the best way to minimize unsightly colors is to make sure that white balance is set properly on the camera so that the camera knows the color temperature of the light source(s). We do this by either setting a custom white balance or setting a manual WB setting on the camera. No matter what camera you are using, you will find (lighting) situations where the camera is easily fooled and you'll end up with horrible color casts. Shooting under mixed indoor lighting often fools cameras as do shots of subjects that are biased toward only one or two colors such as green grass or red leaves where there is no white reference in the frame for the camera to "lock" onto. When shooting fall leaves in sunlight, for example, it is best to set your camera's WB to "sunlight" manually. Most cameras assume a "gray world" when trying to calculate white balance automatically and scenes that are predominantly one color or have no neutral/white references in the frame easily fool these AWB algorithms.
There is no substitute for getting WB correct at shooting time and the most accurate method of setting WB is to use the "custom" WB setting provided your camera offers that option. Using custom WB amounts to shooting a white object as a reference and then shooting remaining shots normally. It helps to carry a white piece of paper, white card, or gray card, but other objects can be used as well such as white shirts, a white door, concrete driveway, chrome/silver objects, etc. Normally you are only required to have the white/gray object cover the center metering circle in the camera's viewfinder so the white/gray object need not cover the entire frame. While this may sound impractical, you may be surprised how much of an improvement it can make to your photos. As long as your lighting isn't varying as you shoot, you can pick any object that happens to be under the same lighting as your intended subjects, take a custom WB shot, and then shoot the rest of your shots with peace of mind that WB adjustments will not be necessary later.
Since some objects can often be misinterpreted as true white, be careful about picking objects that might have a slight color cast and may throw off your WB a bit. If using paper to set custom WB, use a plain sheet of copy paper as many high quality photographic papers tend to have brighteners that actually color the paper slightly blue. An idea that may make custom WB easier is to take a heavy weight sheet of white paper and cut a circle to the size of the inside of your lens cap for your camera. Place it inside the lens cap and in a pinch, you can remove the lens cap and hold it a foot or so in front of the camera, then take a shot of the paper in the cap to acquire the custom WB. Check your manual, but remember that most cameras only require that the white reference cover the center metering circle in the viewfinder, making the round paper in the lens cap work nicely for this situation.
It is true that some photographers don't mind "tweaking" photos to get each one just right, but no one likes guesswork and unless you happen to know exactly what light source was being used, guesswork is what you'll be faced with trying to fix WB after the fact. Take the extra few seconds before you shoot to set your white balance appropriately in-camera. Take a few seconds to shoot a white object and set a custom white balance if you have a known white or gray object as a reference and it could save you a lot of frustration trying to fix color problems later. If you shoot in JPEG mode as opposed to raw, getting white balance right up front is even more important as subtle color errors can occur if the JPEG images are developed with the wrong assumption for white balance. Automatic white balance can do a decent job but it is easily fooled by complex lighting conditions or certain subject material and even small errors in WB can cause a significant change to the overall look and feel of a photo. White balance is one shooting parameter that is worth getting right before you shoot!
-- Mike Chaney