Getting White Balance Right.
by eduardoangel – December 15th, 2009
By Joe Brady
In previous posts, we’ve spoken about all the benefits of creating custom profiles with the ColorChecker Passport, but I’d like to back up a bit and talk about the place that white balance plays in getting the best color from your digital camera files.
Simply put, white balance removes color casts from neutral or white parts of an image. Our eyes are great at figuring out what parts of a scene in front of us should be white and our color interpretation of the scene is adjusted for us with our brain’s “Auto White Balance” feature. Unfortunately, cameras can only average the white balance based on the scene and for a truly neutral starting point need to be told what part of the image is “White”.
If color temperature of light is something new to you, let’s do a quick review. We’ll skip the science and get right into a list that compares approximate color temperatures with their appropriate light source. The temperature is in degree Kelvin (K) and the higher the number, the bluer the light.
Color Temperature Source of Light
1000 – 2000K Candlelight and dimmed tungsten lights
2000 – 3500K Household tungsten light bulbs
2500 – 4000K Sunrise and sunset
4000 – 5000K Fluorescent tubes
4800 – 5800K Electronic flash
5000 – 6500K Overhead sun – no clouds
6800 – 8500K Overcast skies
8000 – 12000K Continuous low cloud cover and shade
You may notice that the range in some of these lighting situations can be quite large and we haven’t even considered that artificial light sources don’t put out a continuous spectrum of light within their base color temperature. This in part illustrates one of the problems when you adjust white balance in JPEG and TIFF files. If you choose one of your camera’s preset white balances, a specific color temperature is built in or embedded into the file. If you need to adjust this later, either by eye, or resampling a white balance using a neutral reference card, you are in danger of decreasing the bit depth of your image. This is something to be avoided, as decreased bit depth can cause banding in gradations and loss of detail in shadows and highlights.
The best solution is to always use a Raw workflow with your camera. Since there is no white balance embedded in the Raw data, you can assign neutral by creating a custom white balance in camera, or you can sample on a white balance patch using a reference like the ColorChecker Passport or White Balance Calibration Target. Creating a white balance at capture however does offer a few advantages.
First, it can save time when processing files later on. Since most Raw conversion utilities assign whatever white balance was set in the camera at the time of capture, you won’t have to locate a target reference photo to white balance off of. The second advantage has to do the your camera’s LCD screen. Even when shooting Raw, the image preview on the back of your camera is a small JPEG that has been processed from your Raw capture. This file uses whatever white balance is currently set in the camera to build its image preview. If you have a correct custom white balance set before you start shooting, the previews on your camera will be more accurate.
You can’t simply choose a white piece of paper or clothing as a white balance reference because these items are rarely pure white. Both typically have colorants and additives that reflect light differently under different light sources, so using them for a white balance will often keep you from getting a true neutral. The ColorChecker White Balance Targets are spectrally neutral, meaning they reflect red, green and blue equally under the light sources you will normally encounter. This insures a correct white balance for your images.
Having a correct white balance is also critical for the best results with custom camera profiles. Camera profiles adjust for the difference between colors as measured off a capture of a ColorChecker target. They still need an accurate white balance first in order to give you back the best and most accurate color from your photographs. Get your white balance set first, then create a camera profile with the ColorChecker Passport software, and you will be amazed at the color in your photographs. We’ll explore this in more detail in future posts.