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Wednesday, March 3, 2010

Why Snow Looks Blue

It has been a cold, snowy winter here in North Carolina. With all the frozen flakes of white(even here in ENC), photographers are shooting plenty of snowy scenes. This brings me to a question I got recently:

Q: I took a picture of a snow scene and it turned out with a bluish cast. What causes that? How can I fix it so it will look natural not blue?

I've experienced the same problem especially during cold weather in low light conditions. To understand what's going on and, of course, how to correct it, first you need to know a little about visible light. Let's start with some basic physics. Hold on. This won't be a elaborate technical discussion from a science textbook. It is however, important to know something about how light behaves. In photography, after all light is the first element of creation. The more you know, the more you can practice good photographic techniques. I'll keep the technical stuff to a minimum.

Lighting, Camera, Action!

Visible light (what your eyes see) is composed of different wavelengths or color spectrum. Violet light is at one end of the spectrum with red at the other. You can see these different colors in a rainbow or when sunlight is refracted by a glass prism (see image below).

Refracted light by a prism

The spectrum of colors are said to have a color temperature measured in degrees Kelvin(K), and refer to the relative warmth or coolness of white light. Light with a higher color temperature (larger Kelvin value) has more bluish tones while light with a lower color temperature (smaller Kelvin value) had more reddish tones. I included the chart below as reference so readers will have a idea of what the numeric temperatures mean in real life situations.

Why are the colors of light measured as a temperature? In the late 1800s British physicist Lord Kelvin heated a block of carbon until it glowed. As the carbon got hotter, it produced a range of different colors. The black cube first produced a dim red light, increasing to yellow as the temperature rose. Eventually it glowed with a bright blue-white at the highest temperature. In his honor, color temperatures are measured in degrees Kelvin. For a more in-depth discussion check out What is Color Temperature? by Jim Zuckerman.

Color Temperature in Photography

Now with the technical details out of the way, let's focus on the application of color temperature in photography. A digital camera has a sensor which captures the incoming light from a subject then records the image. The problem is the camera doesn't always capture what we see as “natural coloration”. The color temperature or white balance simply doesn't appear like our eyes see it. Hence some images come out looking the wrong color. So what is white balance(WB)?

A simple explanation is WB is the process of removing unnatural color casts, so that subjects which appear white by our eyes are rendered white in the photo. Our eyes are very good at judging what is white under various light sources, however digital camera sensors sometimes get it wrong especially when shooting in the incorrect WB mode. A wrong WB can result in pictures turning out with a unsightly yellow, orange or blue cast. A prime example is a winter snow scene with the unwelcome bluish tinge (see Figure 1). Sometimes, the blue adds to the mood of the picture, but at other times you may want it to look natural as your eyes see it.

Figure 1
snow scene shot in Daylight WB

Here's how to make the blue cast disappear.

First step is to consult your camera's manual and determine how to change the WB. This will vary from model to model. It's important to become familiar with all the WB settings so that you can quickly change them as needed. Shoot the scene at several different WB settings. Cameras with live preview will show on the LCD screen how the image will appear before you take the picture. For guidance on which to use, see the chart below. 

WB ModeLighting Conditions
Camera's best guess for a color temperature between 3000-4000K and at 7000K
Outdoors with overcast skies, shady areas or twilight skies
Outdoors on a clear day or to capture the reds in a sunset or colors in fireworks
Indoors with incandescent lighting. Use outdoors for enhanced blue effect
Optimal white balance setting when camera is pointed at a solid white object(see note)

Note: some cameras allow the shooter to set a custom WB. Consult your manual for details.

For Easy Color Correction Shoot RAW

Many cameras allow the shooter to save images in more than one format: JPEG and/or RAW. Referred to as a digital negative, RAW gives you much more flexibility. When you save an image in RAW file format, you are saving it the way the image sensor sees it without applying any adjustments including white balance. In fact, the camera ignores the WB settings. Bottom line: with RAW you can correct mistakes made in the field. Let's say you get home from a trip only to discover your images have the wrong color. You can correct the error in post processing.

Image Editing With Photoshop Elements

Here is a short tutorial on how to remove a blue color cast. It's by no means detailed instructions on all the post editing features. I'll save that for another article. Using Figure 1 as an example, open your image in Photoshop Elements. The image has an unsightly blue cast we want to remove. On the right panel you will see White Balance with a drop down menu and below that a Temperature line with Kelvin values. The current WB is “Daylight” which corresponds to a temperature of 5500K.

Figure 2
RAW image in PSE with blue cast

By changing the WB you can correct the color cast to a more normal, realistic look. With a bit of practice you can achieve just what your eyes really saw. Below is the same image with the custom WB temperature adjusted to 6200K. Notice the blue cast is gone.

Figure 3
RAW image in PSE with custom WB
With a couple of tweaks(exposure, contrast, clarity, etc) the snowy scene looks real-to-life. So next time you are out in the snow, remember to shoot, shoot, shoot and experiment with different settings. You may not have the opportunity to capture the same scene again(at least not this year).

  Figure 4
Final image with WB correction
Color Cast Correction of JPEGs

With JPEGs, you can remove unwanted color casts with Remove Color Cast found under the Enhance-->Adjust Color menu. A box with a eyedropper will appear. Inside the box, you will see instructions on removing the unwanted cast. The key is clicking on area of gray, white or black then see if the cast changes to a natural look. This technique is more trial and error so it will take a few attempts to get it right. Note: sometimes it is very hard to get a JPEG color cast exactly correct. Unlike RAW files, JPEGs have the shot settings(WB, exposure, etc) saved into the file. It's not always easy to undo what's already saved in the file.

Figure 5
JPEG image after using Remove Color Cast

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All images and content are ©2010 Kelvin Taylor


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