Sunday, May 7, 2017

Auto White Balance is not What You Think

Auto White Balance is not What You Think

Most photographers tend to set their cameras on auto white balance and assume that everything will be taken care of. Not necessarily.

First off, we need to spend a little to time to review why "white balance" exists at all and what "auto" white balance is intended to solve.

Not all light sources are created equal in that they don't all produce the same color of light. This should seem obvious if you've ever seen a fireworks display or Christmas tree decked out with colored lights. But what can be less obvious is that the color of light received from the sun varies in color throughout the day. As sunlight travels through the atmosphere, the blue portion of the light gets scattered, giving the sky its characteristic hue. At daybreak and at sunset, the sun is at an oblique angle relative to the earth's surface as compared to high noon. This means it has further to travel through the atmosphere early and late in the day, and thus more blue light gets scattered. The golden light of sunrise and sunset is a direct result of this reduction of the blue portion of the spectrum. Nobody added more orange at sunset, it's just that more blue has been scattered, leaving a greater proportion of orange. Light on a cloudy day has a blue cast to it since the predominant source of light then is that diffused blue light without any direct sunlight. Old style incandescent light bulbs indoors have a distinctly orange cast as anyone whoever shot slide film indoors knows. It only takes one roll of botched birthday party photos to learn that lesson. Every light source imparts its own color temperature cast to the light it produces.

When we human beings look at something, our eyes send a signal to our brains. We don't really see that our eyes see, we see what our brain interprets that signal as. One of the magical things our brains handle for us is the automatic correction of color cast imparted by the prevailing light source. In most cases, we see the color we expect to see. We assume that paper is white for example, so we automatically correct for anything that makes it not white. Light outdoors over most of the day seems relatively neutral in hue. At sunrise and sunset though, we expect and appreciate the golden light. Magically, it just all works.

But the automatic white balance (AWB) on our cameras doesn't really know what it's looking at nor what color it should be. The general approach for auto white balance on most cameras is to assume that "on average" a scene should be neutral in color. That is, if you average out all the pixels in an image, the result should have no color cast — it should be some neutral shade on the gray scale from black to white. If the average of some image comes out somewhat blue, the camera will "correct" things by shifting colors sufficiently towards orange, and vice versa. This allows you to photograph many scenes without worrying about color temperature, since that assumption about the average scene really does work much of the time.

But not always. When photographing people near sunrise or sunset, you probably don't want them to appear orange, but when photographing sunset itself, you generally are doing so specifically because that warm orange hue is itself beautiful. Everyone loves a sunset. If your camera cancels out some of that golden glow, even with the best of intentions, you probably won't like it. This is why your camera also give you the ability to directly control white balance.

Here's another example. Try taking a close-up photo of a strongly red or fuchsia flower that fills the frame. Your camera will have a hard time knowing what to do with this and may (or may not) decide to lessen the saturation or shift the color of that flower photo. Different cameras react to such clearly non-neutral-average scenes differently. But most will get confused at least some of the time when set to auto white balance.

Knowing this, some photographers will make their own assessment of white balance and adjust their camera settings accordingly. You may have seen photographers using a gray card for just this reason. You may even be one of those photographers, carrying a gray card with you to every location. Whether this makes sense has a lot to do with what type of images you intend to shoot. The ones who legitimately need to care the most are commercial photographers who need to make sure that the color of the client's logo and packaging comes out with the exact proper shade of red, green or whatever. Wedding photographers also have a strong motivation to get white balance correct. The happy couple won't be so happy if their wedding photos come out with an odd color cast to them. But wedding photographers do have an easy way to avoid needing a gray card in that the groom's tuxedo will generally be black and the bride's gown white. They can simply use those as references to adjust white balance. Nature photographers like me are among the least likely to want any assistance in removing color casts, at least those caused by the progress of the sun through the sky. I want my sunrises to look like sunrises.

Now, all this brings me to an extremely important point. If you shoot in JPEG mode, the setting for white balance will be burned in as part of the final image saved to your camera's memory card. But if you shoot in RAW mode, the value for that setting will merely get saved as metadata alongside the raw pixel data as recorded by your sensor. "Raw" does mean raw. Except for unavoidable settings such as your choice for aperture and shutter speed (and thereby exposure), the settings choices you make on your camera have no direct result on the image (raw, unprocessed image) you end up with. The values for white balance, saturation, sharpening and many others still get saved with each raw file, but their effect won't alter the pixel values in that file. They become part of the metadata that accompanies the image.

Most raw conversion programs including Lightroom and Adobe Camera Raw don't directly use the white balance value your camera was set to when you shot an image. Editing it on your computer, you are free to move the white balance sliders any way you want to make the image look the way you think it should. If you tell Lightroom to automatically adjust the white balance for an image, it will employ an "averaging" strategy very similar to what cameras do, but it does so independently, without regard to the metadata white balance. Given this, it really doesn't matter what white balance your camera is set to if you shoot raw. Although the value will get saved with each image's metadata, you can freely change it to anything else you may want without fear of damaging image quality. Until you convert your raw image data to some other format, no white balance has altered the actual saved image data. What you see on your screen is merely a rendering made on the fly so you can see what it would look like.

All this having been said though, there is one thing your camera white balance choice will always influence: the camera LCD image rendering. Even when shooting RAW, there is always an embedded JPEG preview that gets used for this (and other) purposes. As such, it may indeed make sense to set your camera to auto white balance. No, not for the final images you render in Lightroom or some other program on your computer, but solely for the benefit of the LCD image rendering on your camera back. This way, without your ever having to fiddle with white balance on your camera, you will get the benefit of your LCD camera back renderings appearing mostly good, on average. I've found that if an image looks good on the camera back, I'm more likely to shoot more of that subject. I just know that if the LCD version has an explainable color shift, I don't need to worry too much. The final image on my computer or printer can look as it should regardless.

So, using the auto white balance setting may in fact be a good idea. So long as you understand what it does, and what it doesn't do.

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Time Trap Photography is dedicated to freezing those special moments in life that can be revisited and admired for generations to come. - Shannon Bourque

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