白平衡教程翻译.doc

TUTORIALS: WHITE BALANCE白平衡教程
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, however digital cameras often have great difficulty with auto white balance (AWB).  An incorrect WB can create unsightly blue, orange, or even green color casts, which are unrealistic and particularly damaging to portraits.  Performing WB in traditional film photography requires attaching a different cast-removing filter for each lighting condition, whereas with digital this is no longer required.  Understanding digital white balance can help you avoid color casts created by your camera's AWB, thereby improving your photos under a wider range of lighting conditions.
白平衡是消除不真实的色彩偏差的过程，使得原本是白色的物体在照片中也再现同样的白色。正确的相机白平衡必须要考虑到光源的“色温”，色温指的是光线的相对“冷暖”。人的眼睛可以很容易的在不同光源下辨认出哪个是白色，但数码相机在自动白平衡的时候往往要面临巨大的困难。不正确的白平衡将导致难看的蓝色、橙色甚至是绿色的色偏，这种不真实对于肖像照片来说为害更为严重。在传统的胶片摄影中应用白平衡需要为不同的光线条件附加不同的偏色消除滤镜，而在数码摄影中就完全不需要了。理解数码的白平衡有助于你避免相机的自动白平衡偏色，在各种光照条件下都可以拍出好的照片。

Incorrect White Balance
Correct White Balance
BACKGROUND: COLOR TEMPERATURE 背景知识：色温
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:
色温描述的是一个具有某种表面温度的“黑体”所辐射出的光谱范围。“黑体”是一种吸收全部入射光的物体——绝不反射或是穿透任何光线。在我们的日常生活中，可以举出类似的物体，例如加热一块金属或是石头：当它们加热到一定温度时，它们就变得“红热”，然后继续加热，它们就变成“白热”。同样的，不同温度的黑体发出的“白色光”也有着不同的色温。虽然都叫白色，这些看上去白色的光在可见光谱中并不总是包含均匀分布的色彩。

Relative intensity has been normalized for each temperature (in Kelvins).
每种温度的相对强度已经被规范化了（开氏度）
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.
注意看5000k 的曲线更近似于中性色，而3000k和9000k产生的光谱会分别偏向于橙色和蓝色。随着色温的增高，色彩的分布就变得越冷。这并不符合我们的直觉体验，但该结果基于这样一个事实：波长越短，包含的光能越高。
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:
为什么对于摄影师喜欢用色温来描述光，即便他们从未与真正的“黑体”打交道？很幸运，日光和钨丝灯之类的光源很近似于黑体的光谱表现，但其他的光源，如荧光灯和大多数商业光源就与黑体大相径庭了。由于摄影师们从不使用“色温”这个术语来描述真正的黑体光源，这个术语暗含着近似色温黑体的“正确色温”的意思。下表是一些常见光源的正确色温设置的首要原则。
Color Temperature 色温
Light Source 光源
1000-2000 K
 Candlelight 烛光
2500-3500 K
 Tungsten Bulb (household variety) 钨丝灯（家用）
3000-4000 K
 Sunrise/Sunset (clear sky) 日出/日落（晴天）
4000-5000 K
 Fluorescent Lamps 荧光灯
5000-5500 K
 Electronic Flash 闪光灯
5000-6500 K
 Daylight with Clear Sky (sun overhead) 晴天日光（当顶）
6500-8000 K
 Moderately Overcast Sky 中度阴天
9000-10000 K
 Shade or Heavily Overcast Sky 阴影或重度阴霾天气
IN PRACTICE: JPEG & TIFF FILES 实践：JPEG 与 TIFF 格式
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.
由于一些光源并不类似黑体辐射，白平衡在色温之外还采用了第二种方式：绿紫漂移。在普通日光下是不需要绿紫漂移的，但荧光灯和其他人造光源就需要有效的绿紫调节才能达到白平衡。

Auto White Balance
自动白平衡

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.
幸运的是，大部分数码相机都包含一系列的预设白平衡，你就不需要拍摄关头手工设置色温和绿紫漂移了。通常会采用左边这样的图标。
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.
前三个白平衡允许一定范围的色温。所有数码相机都使用自动白平衡，通过一种最佳猜测算法——通常限制于3000/4000 K 至7000 K之内。自定义白平衡让你在当前光照下拍摄一张已知的灰色参照物，以此作为之后照片的白平衡基准。开尔文（Kelvin）可以在较大的范围内设置色温。
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.

Custom
自定义白平衡

Kelvin
开尔文色温

Tungsten
钨丝灯

Fluorescent
荧光灯

Daylight
日光

Flash
闪光灯

Cloudy
阴天

Shade
阴影
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.
下面的六种白平衡按色温增加的顺序排列，不过许多数码相机并不包含阴影白平衡。一些相机还包括荧光灯H的设置，用于新型的日光校准的荧光灯。上述的白平衡名称与图标只是对它们最适用的实际光照的粗略表示。实际上，阴天白平衡也可以用于日光下，取决于在一天中的哪个时段，以及海拔、雾的浓度等。总之，假如你的照片在液晶屏幕预览时看起来太冷（不管设置如何），你就可以选择列表中的下一个图标来快速增加色温。假如图像还是太冷（或者在其他方向时候太暖），你就可以在开尔文设置中手动输入一个温度值。
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.
假如所有的手段都失败了，最后照片在显示器上显示的还是不正确的白平衡，你可以调节色彩平衡，去除多余的偏色。或者，我们可以在Photoshop的“色阶”面板中用“设置灰点”吸管来点击一个色彩参照物（见自然参照物一章）。但这些手段都是应该尽量避免的，因为都会严重的损失图像的色彩位深。
IN PRACTICE: THE RAW FILE FORMAT 实践：RAW格式
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.
到目前为止，最好的白平衡方案是使用 RAW 格式拍摄（当然，需要你的相机支持这种格式），这样你就可以在照片拍摄之后再来设置白平衡。此外，RAW 文件的白平衡设定还允许用户进行非常大范围的色温和绿紫漂移设定。
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).
在 RAW 文件中设置白平衡非常的快速和简易。你可以调节色温或绿紫漂移滑块，直至色偏完全消除，或者你可以在照片中的一个自然参照物上简单的点击一下（见下一节）。即便你的这一系列照片中只有一张包含有自然参照物，你一样可以点击它，然后将白平衡设定的结果应用到其他照片上（假定它们都是同样的光照条件）。
CUSTOM WHITE BALANCE: CHOOSING A NEUTRAL REFERENCE
自定义白平衡：选择一个自然参照物
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.
白平衡参照物通常用于一些对色彩要求极其严苛的物体，或是拍摄者预计自动白平衡将遇到麻烦的情形。自然参照物可以是你场景中的一个部分（希望你好运），或是一个你可以随身携带的物件。下面是一个黄昏场景的例子，假如不是其中有一个幸运的参照物，这个场景就会严重的偏蓝色。

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:
在其他情况下，预先制作一个可以携带的参照物或许要精确得多，因为人眼也很容易被欺骗，把一个偏色的物体误认为是自然参照物。可携带的参照物可以很昂贵，例如专业摄影用的灰卡， 也可以是一些不那么昂贵的日常用品。一个理想的灰色参照物应该能够同等的反射色谱中的全部颜色，并且在一个宽广范围内的各种色温条件下都有恒定的表现。下图显示的是一个预制的灰色参照物的例子：

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).
常见的一个可以充当自然参照物的日用品就是咖啡或品客薯片的盖子内侧。这些都不昂贵并且相当精确，虽然专业的摄影用灰卡是最好的（上图所示的卡片）。专业设备可用来测量光照的入射光或反射光的色温。大部分的自然参照物都可以测量反射光，而一个白平衡计或 "ExpoDisc" 则可以测量入射光（理论上会更精确）。
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:
在使用一个有高iso噪点的自然参照物时要非常小心，虽然你点击的是一个看上去灰色的区域，但可能实际上你选择的是一个彩色噪点造成的彩色像素。

Low Noise 低噪点
(Smooth Colorless Gray)
平滑无色的灰
High Noise 高噪点
(Patches of Color)
彩色色块
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.
如果你的软件支持，在噪点照片上设置白平衡的最佳解决方案是，采用噪点灰色区域的多像素平均值来作为参照物。
NOTES ON AUTO WHITE BALANCE 关于自动白平衡
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.
某些物体会造成数码相机自动白平衡的困扰——即便是在普通的日光条件下，一个例子是某些题材中的图片中出现大范围的暖色或冷色。下面的照片说明的是拍摄物出现大面积红色的情形，因此相机误以为这是一个暖光源造成的色偏。相机就试图对此进行补偿，让照片更接近于自然，但结果却使得石头都罩上了一层蓝色的色偏。某些相机对这种情形会较为敏感。

Automatic White Balance 自动白平衡
Custom White Balance 自定义白平衡
(Custom white balance uses an 18% gray card as a neutral reference.)
（使用一个18% 的灰卡作为自然参照物来定义白平衡。）
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.
当照片中包含至少一个白色或亮灰色物体时，数码相机的自动白平衡更容易有效工作。当然，不需要刻意改变构图加入一个无色物体，只需要留意到它的缺失将会给自动白平衡带来麻烦。下图中如果没有那个白色的小艇，相机的自动白平衡就会错误的制造出稍微偏暖色的色温。

IN MIXED LIGHTING 混合光照
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.
不同色温的多个光源将让白平衡设置变得更为复杂。在某些光照场合下也许永远不会有什么真正“正确的”白平衡，完全取决于哪种颜色的精确性更为重要。

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.
在混合光照情况下，自动白平衡通常为整个场景计算出平均色温，然后以此作为白平衡。这种方式通常是可行的，然而和人眼所见相比起来，自动白平衡倾向于夸大每一个光源的色温差异。
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.
这种夸大常见于室内光线与自然光混合的场景。某些重要图片甚至需要为每块光照区域设置不同的白平衡。其次，某些场景还是保留原来色温更好。
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.
注意左边的建筑非常温暖，而天空却很冷。这是由于白平衡设置是基于月光，使得下方的人造光源变暖。基于自然光照的白平衡通常会得到更真实的照片。假如选择“石头”来作为白平衡参照物，天空将会蓝到夸张的地步。
Reference:
参照物：
Moon
月亮
Stone
石头

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中文翻译：snlchina@