Wednesday, March 4, 2009

A Tale of Two Color Casts



Light is made up of different colors. Even what our eyes might see as "white" light can be a range of colors. For example, the color of sunlight varies throughout the day. It has a golden/reddish/warm tone to it during sunrise and sunset. It has more of a blue cast during high noon. Have you even taken a photograph indoors and wondered why everything had a yellow to orange color cast to it? That is because we typically use tungsten light bulbs to illuminate our homes. The tungsten filaments in the bulbs emit light that is more to the yellow or orange side of white than toward the blue side of white. Fluorescent lights can cause a green cast. Sodium vapor, mercury vapor or other outdoor lighting have a whole host of other color casts to them. Our eyes are remarkable optical instruments and can adjust to these color casts so that we perceive white light where there may actually be another color in the light. An easy way to see this effect is when you are driving down the interstate at night. Look at the headlights of the on-coming traffic. You will likely notice that most of the cars' headlights have a slight yellowish tint to them. Then you will see some really "bright" headlights, usually BMWs or Mercedes Benz, that look more blueish or more "white" than the other cars. Those cars have the more expensive headlights that burn at a brighter "color temperature" than the other cars which gives them the bluer color. Those bluer lights are not actually any brighter than the normal headlights. Our eyes are just more sensitive to the blue portion of the spectrum than the yellow part of the spectrum. Thus, the bluer headlights appear "brighter" to us simply because our eyes are designed to collect more blue light than yellow light.

This difference in colors of "white" light is what is called white balance in photography. Our cameras only see the light that enters through the shutter. They will record light with what ever color cast might be present. This is why your indoor photographs turn out with yellow or orange color casts to them. The camera's "white balance" wasn't set to consider the tungsten light "white". So instead it recorded it with the color cast you see in your photograph. Modern digital cameras can be adjusted on the fly to whatever "color" light you might be working in via the camera's white balance setting. Digital photographs, especially those shot in a RAW format, can be adjusted to a proper white balance during post processing. When using film cameras, you have to select the proper kind of film that is balanced to the kind of light you will be shooting in. If the light changes, you would have to change your film or your subsequent photographs will have odd color casts to them.

Which of the photographs above was the "natural" light photograph, and which was the color adjusted photograph? Find out after the jump.

Near my neighborhood is a small parking lot with several sodium vapor lights that illuminate it at night. They make it hard to see stars at night from our house. They also cause a color cast when you try to photograph at night. The color cast becomes exaggerated during bad weather such as fog, rain, ice or the snow we had a couple of days ago. In fact, once there is a coating of snow on everything, the color cast gets amplified by reflecting off of all of that white snow and causes everything you see to turn into a sepia tone. Thus, the first photograph above called "Sepia" is actually the "natural" light version. That color is what I saw when I was outside that night. It is a really weird feeling walking around a naturally sepia toned world. Almost like walking around in an old silent film or Civil War era photograph. However, it is also a really interesting light because it is so different than what we usually have to work with photographically.

The second photograph is the very same image after I color corrected it to a more neutral or white color tone in Photoshop. It almost looks like it was shot during the day, doesn't it? The truth of the matter is that both photographs were taken on my tripod with a four second exposure time. The amount of light in the image which is due to both the exposure time and all of the snow-covered reflective surfaces as well as the snow in the air makes for an optical illusion that the image was taken during the day. The neutral color cast adds to this illusion because we expect to "see" this kind of white light during the day, not at night.

While photographers generally strive to get the light to a neutral tone in order to eliminate undesirable color casts, said color casts can also be used creatively for mood and effect. Here, the "natural" sepia tone of the sodium vapor illuminated snow invokes a much different mood than the color balanced version. Which is best? It depends on what the photographer means to convey in the image. I took the image to experiment with the odd sepia ambient light and to compare it to a color balanced version. In the end, I like both images although I am more partial to the natural sepia toned version in this instance. It invokes a stronger mood to me than the color neutral version does. Which do you prefer?

I have to admit, having a source of "natural" sepia lighting like this could have some interesting potential. Plus, walking around in a sepia world is very unique.

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