Sunday, March 4, 2012

Introduction to White Balance for Digital Camera Users (Part I)

Apollo/Saturn V Center at Kennedy Space CenterI'm often asked about white balance by travelers using digital cameras who periodically have a noticeable and sometimes severe color cast on their travel photos. More often than not I'm queried about what white balance actually is, why “auto white balance” doesn't work, and how to get the color rendition of a scene “right.”

It turns out that white balance is a difficult term to define for most people. Here's my try.
White balance is the process of adjusting color casts, so that objects which appear white to human eyes/brains are rendered white in the photograph by the camera.
I'm sure you're asking what the heck that means.

Photography is all about light. If we examine light, which from the sun appears white to our eyes, we find it's made up of a whole spectrum of colors, each with its own color temperature which describes its individual color. Cameras need the ability to render the colors of scenes as the human eye sees them, and that isn't necessarily easy.

In the past with film photography, the effect of the color temperature of the light falling on the photos' subject was controlled by film choice. Film was purchased, in part, with consideration for the particular light illuminating the scene, daylight, vs. tungsten, for example (outdoor vs. indoor). Each type of film was physically and chemically designed to render a specific light range of color temperatures accurately.

In the world of digital photography, the choice we make as photographers for white balance, controls the camera's accuracy in rendering the specific range of color temperatures of the light illuminating each scene.

Moreover, as black and white photography is rarely used by the majority of digital photographers today, other than generally pros and enthusiasts, color rendition has become more important than ever. (Actually color rendition is important in Black and White photography too, as various shades of gray are renderings of the colors of the scene.)
It turns out that the color rendition of a scene and its white balance go hand in hand.
It is important to immediately note, that in some scenes something considered white will not appear white to our eyes all the time. During the “golden hours,” for example, white can appear “golden.” At sunset white buildings can take on red and golden hues. To render them that way, instead of white, would certainly be correct.

So, when examining my definition above, please note white balance is defined in terms of the human perception of color. White balance is all about rendering the colors of a scene well, and that's all about the photographer's visual perception of the scene.
The closer your white balance is reflective of the scene's color temperature, the closer the color in your photograph will be true to life.
Each light source has its own color temperature, but how it's used and controlled can affect its color temperature when it illuminates a scene. For example, the light coming from the sun at different times of the day has different color temperatures. The changing color of the sun over the course of the day is mainly a result of scattering of light. (You didn't think you were going to get a physics lesson, did you?)

Think about how a scene looks at noon vs. at sunset. Moreover, the color temperature of the sun in bright daylight is different than when it's filtered by light clouds, and different again when filtered by heavy clouds.

In each case while your eyes/brain know what's white and will make white look white for you, when possible, and according to your perception, the camera can't do that without some kind of circuit changing the color rendition produced by the sensor, and that's the camera's white balance control circuit.

Different scenes have different generally recognized color temperatures, expressed in degrees Kelvin, which describes the light tone of the scene. Here are some basic, typical color temperatures of scenes we often photograph:
  • Candlelight 1,500k
  • Incandescent: 3,000k
  • Sunrise/Sunset: 3,500k
  • Fluorescent: 4,500k
  • Direct Sun: 5,500k
  • Electronic Flash: 5,500k
  • Cloudy: 6,500k
  • Shade: 8,500k
It turns out that “auto white balance” handles many different scenes pretty well, but does have general limits in DSLRs of about 4,200k to about 7,000k. For most cameras, you can find their “auto white balance” range specification in your camera's manual. My cameras' “auto white balance” range is 3,500–8,000k, a range larger than typical. Check to see precisely what your camera's “auto white balance” capability is, as the “auto white balance” capability of digital cameras continues to improve with new digital camera offerings.

So, generally from about fluorescent light through light clouds “auto white balance” works pretty well, though at the limits of “auto white balance” it doesn't balance the white as well as when in direct sun daylight. Even my professional Nikon camera has definite limits in its “auto white balance” capability.

Now that you understand the concept of white balance, next week in Part II, I'll discuss how to set the white balance setting in your camera to eliminate, or at least mitigate white balance problems when you make your digital photographs.


黄清华 Wong Ching Wah said...

Thanks for some explanation on white balance !

Ned S. Levi said...

Thanks for your comment. I appreciate your kind words.

Come on back next week for Part II. It has the real guts of the overall article.



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