Back in the film days, color films were standardized, or balanced, for a particular "color" of light. Digital cameras like the EOS 40D use a particular "white balance" matched to the color of light used to expose your photograph. The right white balance is measured using a scale called color temperature. Color temperatures were assigned by heating a mythical "black body radiator" and recording the spectrum of light it emitted at a given temperature in degrees Kelvin. So, daylight at noon has a color temperature in the 5,500 to 6,000 degree range. Indoor illumination is around 3,400 degrees. Hotter temperatures produce bluer images (think blue-white hot) while cooler temperatures produce redder images (think of a dull-red glowing ember). Because of human nature, though, bluer images are actually called "cool" (think wintry day) and redder images are called "warm," (think ruddy sunset) even though their color temperatures are reversed.
If a photograph is exposed indoors under warm illumination with a digital camera sensor balanced for cooler daylight, the image will appear much too reddish (see Figure 5.18). An image exposed outdoors with the white balance set for incandescent illumination will seem much too blue (see Figure 5.19). These color casts may be too strong to remove in an image editor from JPEG files, although if you shoot RAW or sRAW you can change the WB setting to the correct value when you import the image into your editor.
The Auto White Balance (AWB) setting, available by pressing the Metering-WB button and spinning the Quick Control Dial, examines your scene and chooses an appropriate value. However, the 40D's selection process is far from foolproof. Under bright lighting conditions, it may evaluate the colors in the image and still assume the light source is daylight and balance the picture accordingly, even though, in fact, you may be shooting under extremely bright incandescent illumination. In dimmer light, the camera's electronics may assume that the illumination is tungsten, and color balance for that. Fortunately, some electronic flash
units, such as the Canon Speedlite 580EX, can report to the camera the particular white balance that they are outputting, since a flash's color temperature can vary depending on how brief the flash exposure is.
The other presets in the WB list apply specific color temperatures. For example, the Daylight setting sets WB to 5,200K, while the Shade setting uses a much bluer 7,000K. The chief difference between direct daylight and shade or even tungsten light sources is nothing more than the proportion of red and blue light. The spectrum of colors is continuous, but is biased toward one end or the other.
However, some types of fluorescent lights produce illumination that has a severe deficit in certain colors, such as only particular shades of red. If you looked at the spectrum or rainbow of colors encompassed by such a light source, it would have black bands in it, representing particular wavelengths of light that are absent. You can't compensate for this deficiency by adding all tones of red. That's why the White Fluorescent Light setting of your 40D may provide less than satisfactory results with other kinds of fluorescent bulbs. If you take many photographs under a particular kind of non-compatible fluorescent light, you might want to investigate specialized fluorescent light filters for your lenses, available from camera stores.
However, you might also get acceptable results using the final two choices on the WB list. Custom and Color Temperature allow you to use specific white balances you've set yourself using the Custom WB and Color temp choices in the Shooting menu. As I described in Chapter 3, to set a custom white balance you'll need to focus manually (with the lens set on MF) on a plain white or gray object, such as a card or wall, making sure the object fills the spot metering circle in the center of the viewfinder. Then, take a photo under the lighting conditions you want to use as your custom white balance.
Then, press the Menu button and select Custom WB. Rotate the Quick Control Dial until the reference image you just took appears and press the Set button to store the white balance of the image as your Custom setting. The setting will remain in your 40D's memory. To activate it, just press the Metering-WB button and spin the Quick Control Dial until the Custom setting is selected.
As I mentioned in Chapter 3, if you already know the color temperature of your illumination, you can set an exact value in degrees Kelvin using the Color Temp. entry in the White Balance menu in the Shooting 1 tab. Select this menu entry, press Set, and then turn the Main Dial until the color temperature you want, from 2,800K to 10,000K (in 100K increments) is dialed in. (See Figure 5.20.) Then, you can use this setting by pressing the Metering-WB button, and spinning the Quick Control Dial until the Color Temperature choice (the K icon) is selected.
Finally, as I described in Chapter 3, you can shift the 40D's standard color temperature that is used as a basis for all its corrections, changing the default temperature to a new value with a bias towards the green, magenta, blue, or amber directions, which correspond to movements along the central axes of the chart shown in Figure 5.21 in the up (green), down (magenta), blue (left), or amber/yellow (right) directions. You can also combine biases of adjacent hues by moving the cursor diagonally towards the upper left (more blue and more green), upper right (more green and more amber), lower right (more amber and more magenta), and lower left (more magenta and more blue).
If you know the specific color temperature of your light source, you can dial it in.
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