White balance (WB) is the process of removing unrealistic color
casts, so that objects which appear white in person are rendered white
in your photo. Proper camera white balance has to take into account the
"color temperature" of a light source, which refers to the relative
warmth or coolness of white light. Our eyes are very good at judging
what is white under different light sources, but digital cameras often
have great difficulty with auto white balance (AWB) — and can create
unsightly blue, orange, or even green color casts. Understanding digital
white balance can help you avoid these color casts, thereby improving
your photos under a wider range of lighting conditions. |
| Daylight White Balance |
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| Color Cast |
BACKGROUND: COLOR TEMPERATURE
Color temperature describes the spectrum of light which is radiated
from a "blackbody" with that surface temperature. A blackbody is an
object which absorbs all incident light — neither reflecting it nor
allowing it to pass through. A rough analogue of blackbody radiation in
our day to day experience might be in heating a metal or stone: these
are said to become "red hot" when they attain one temperature, and then
"white hot" for even higher temperatures.
Similarly, blackbodies at
different temperatures also have varying color temperatures of "white
light." Despite its name, light which may appear white does not
necessarily contain an even distribution of colors across the visible
spectrum:
Relative intensity has been normalized for each temperature (in Kelvins).
Note how 5000 K produces roughly neutral light, whereas 3000 K and
9000 K produce light spectrums which shift to contain more orange and
blue wavelengths, respectively. As the color temperature rises, the
color distribution becomes cooler. This may not seem intuitive, but
results from the fact that shorter wavelengths contain light of higher
energy.
Why is color temperature a useful description of light for
photographers, if they never deal with true blackbodies? Fortunately,
light sources such as daylight and tungsten bulbs closely mimic the
distribution of light created by blackbodies, although others such as
fluorescent and most commercial lighting depart from blackbodies
significantly. Since photographers never use the term color temperature
to refer to a true blackbody light source, the term is implied to be a
"correlated color temperature" with a similarly colored blackbody. The
following table is a rule-of-thumb guide to the correlated color
temperature of some common light sources:
| Color Temperature | Light Source |
| 1000-2000 K | Candlelight |
| 2500-3500 K | Tungsten Bulb (household variety) |
| 3000-4000 K | Sunrise/Sunset (clear sky) |
| 4000-5000 K | Fluorescent Lamps |
| 5000-5500 K | Electronic Flash |
| 5000-6500 K | Daylight with Clear Sky (sun overhead) |
| 6500-8000 K | Moderately Overcast Sky |
| 9000-10000 K | Shade or Heavily Overcast Sky |
IN PRACTICE: JPEG & TIFF FILES
Since some light sources do not resemble blackbody radiators, white
balance uses a second variable in addition to color temperature: the
green-magenta shift. Adjusting the green-magenta shift is often
unnecessary under ordinary daylight, however fluorescent and other
artificial lighting may require significant green-magenta adjustments to
the WB.
|
Auto White Balance | |
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Custom | |
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Kelvin | |
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Tungsten |
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Fluorescent | |
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Daylight | |
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Flash | |
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Cloudy | |
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Shade |
Fortunately,most digital cameras contain a variety of preset white
balances, so you do not have to deal with color temperature and
green-magenta shift during the critical shot. Commonly used symbols for
each of these are listed to the left.
The first three white balances allow for a range of color temperatures. Auto white balance
is available in all digital cameras and uses a best guess algorithm
within a limited range — usually between 3000/4000 K and 7000 K. Custom white balance
allows you to take a picture of a known gray reference under the same
lighting, and then set that as the white balance for future photos. With
"Kelvin" you can set the color temperature over a broad range.
The remaining six white balances are listed in order of increasing
color temperature, however many compact cameras do not include a shade
white balance. Some cameras also include a "Fluorescent H" setting,
which is designed to work in newer daylight-calibrated fluorescents.
The description and symbol for the above white balances are just
rough estimates for the actual lighting they work best under. In fact,
cloudy could be used in place of daylight depending on the time of day,
elevation, or degree of haziness. In general, if your image appears too
cool on your LCD screen preview (regardless of the setting), you can
quickly increase the color temperature by selecting a symbol further
down on the list above. If the image is still too cool (or warm if going
the other direction), you can resort to manually entering a temperature
in the Kelvin setting.
If all else fails and the image still does not have the correct WB
after inspecting it on a computer afterwards, you can adjust the color
balance to remove additional color casts. Alternatively, one could click
on a colorless reference (see section on neutral references) with the
"set gray point" dropper while using the "levels" tool in Photoshop.
Either of these methods should be avoided since they can severely reduce
the bit depth of your image.
IN PRACTICE: THE RAW FILE FORMAT
By far the best white balance solution is to photograph using the RAW file format
(if your camera supports them), as these allow you to set the WB
*after* the photo has been taken. RAW files also allow one to set the WB
based on a broader range of color temperature and green-magenta shifts.
Performing a white balance with a raw file is quick and easy. You can
either adjust the temperature and green-magenta sliders until color
casts are removed, or you can simply click on a neutral reference within
the image (see next section). Even if only one of your photos contains a
neutral reference, you can click on it and then use the resulting WB
settings for the remainder of your photos (assuming the same lighting).
CUSTOM WHITE BALANCE: CHOOSING A NEUTRAL REFERENCE
A neutral reference is often used for color-critical projects, or for
situations where one anticipates auto white balance will encounter
problems. Neutral references can either be parts of your scene (if
you're lucky), or can be a portable item which you carry with you. Below
is an example of a fortunate reference in an otherwise bluish twilight
scene.
On the other hand, pre-made portable references are almost always
more accurate since one can easily be tricked into thinking an object is
neutral when it is not. Portable references can be expensive and
specifically designed for photography, or may include less expensive
household items. An ideal gray reference is one which reflects all
colors in the spectrum equally, and can consistently do so under a broad
range of color temperatures. An example of a pre-made gray reference is
shown below:
Common household neutral references are the underside of a lid to a
coffee or pringles container. These are both inexpensive and reasonably
accurate, although custom-made photographic references are the best
(such as the cards shown above). Custom-made devices can be used to
measure either the incident or reflected color temperature of the
illuminant. Most neutral references measure reflected light, whereas a
device such as a white balance meter or an "ExpoDisc" can measure
incident light (and can theoretically be more accurate).
Care should be taken when using a neutral reference with high image noise, since clicking on a seemingly gray region may actually select a colorful pixel caused by color noise:
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| High Noise (Patches of Color) |
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| Low Noise (Smooth Colorless Gray) |
If your software supports it, the best solution for white balancing
with noisy images is to use the average of pixels with a noisy gray
region as your reference. This can be either a 3x3 or 5x5 pixel average
if using Adobe Photoshop.
NOTES ON AUTO WHITE BALANCE
Certain subjects create problems for a digital camera's auto white
balance — even under normal daylight conditions. One example is if the
image already has an overabundance of warmth or coolness due to unique
subject matter. The image below illustrates a situation where the
subject is predominantly red, and so the camera mistakes this for a
color cast induced by a warm light source. The camera then tries to
compensate for this so that the average color of the image is closer to
neutral, but in doing so it unknowingly creates a bluish color cast on
the stones. Some digital cameras are more susceptible to this than
others.
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| Custom White Balance |
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| Automatic White Balance |
(Custom white balance uses an 18% gray card as a neutral reference.)
A digital camera's auto white balance is often more effective when
the photo contains at least one white or bright colorless element. Of
course, do not try to change your composition to include a colorless
object, but just be aware that its absence may cause problems with the
auto white balance. Without the white boat in the image below, the
camera's auto white balance mistakenly created an image with a slightly
warmer color temperature.
IN MIXED LIGHTING
Multiple illuminants with different color temperatures can further
complicate performing a white balance. Some lighting situations may not
even have a truly "correct" white balance, and will depend upon where
color accuracy is most important.
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| Reference: Moon |
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| Reference: Stone |
Under mixed lighting, auto white balance usually calculates an
average color temperature for the entire scene, and then uses this as
the white balance. This approach is usually acceptable, however auto
white balance tends to exaggerate the difference in color temperature
for each light source, as compared with what we perceive with our eyes.
Exaggerated differences in color temperature are often most apparent
with mixed indoor and natural lighting. Critical images may even require
a different white balance for each lighting region. On the other hand,
some may prefer to leave the color temperatures as is.
Note how the building to the left is quite warm, whereas the sky is
somewhat cool. This is because the white balance was set based on the
moonlight — bringing out the warm color temperature of the artificial
lighting below. White balancing based on the natural light often yields a
more realistic photograph. Choose "stone" as the white balance
reference and see how the sky becomes unrealistically blue.
source: cambridgeincolour
