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Tuesday, August 20, 2013

Blog Post for August 20, 2013 by John D. Roach

Recently I gave a presentation to my local camera club on some of the basic principals of Photography White Balance.  The following represents the outline slide presentation:

Photographic White Balance
An Introduction
What, How and Why?

What is Color?

Color is caused by differing qualities of light striking an object being reflected or emitted off the object.
To see color, you have to have light. When light shines on an object some colors bounce off the object and others are absorbed by it. Our eyes only see the colors that are bounced off or reflected.
The sun’s rays contain all the colors of the rainbow mixed together. This mixture is known as white light. When white light strikes a white marker, it appears white to us because it absorbs no color and reflects all color equally.
A black marker absorbs all colors equally and reflects none, so it looks black to us. While artists consider black a color, scientists do not because black is the absence of all color.
In Photography we evaluate “Color” in terms of “Temperature.”  


Planckian Scale

        

Color Spectrum of Light Waves

                                          

Different types of light shine with different color qualities.  Direct sunlight is very blue, while tungsten light is very orange.  One of the amazing characteristics of our eyes is that as we move among all different types of light whether in sunlight or artificial light we still perceive color in the same way.
To understand color we must think of it in terms of color temperature using the Kelvin Scale.
If you heat a black object, it will begin to change color.  As it gets hotter, it will progress through the spectrum of colors from red through orange, then yellow, white, and blue white.  The color corresponds to the temperature to which the object has been heated.
NOTE:  Color temperature directly relates to the “how white  balance must be adjusted in a digital camera” so that the correct colors are revealed in the captured light so that it reflects the appropriate colors.
Consider a Bunsen burner, or log fire.  When the fire/flame is at a low temperature, the color of the flame is red.  At higher temperature, the flame changes to bluish tones.  Hot Kelvin temperatures have a bluish tint to the light.
Landscape photographers often recommend shooting during the “magic hours” that surround sunrise or sunset.  During these periods, the light is incredibly “warm” and flattering to the subject. 
The color temperature at sunrise 2000 K and will change rapidly as time elapses.  Only an hour later, the color temperature may rise to 3500 K.  By mid-morning, the color temperature is can be 4300 K.  The “warmer” light of sunrise rapidly yields to the “cooler” light of mid-morning.


RGB Continuum

List of White Balance Settings

Auto:  this where the camera makes its best guess as to quality and type of light.
Tungsten:  Used when incandescent are used indoors in order to cool down colors.
Fluorescent:  Compensates for ‘cool’ light and warms an image.
Daylight: Best for natural light quality during day, sunrise and sunset.
Cloudy:  Warms up a cool effect of a grey day.
Flash: Cool light set to warm up an image.
Shade: Warms up cooling light of being in the shade.


Color Spectrum and Temperature Scale


Now what is White Balance?

White light is composed of every color in the color spectrum.
White Balance is the process of determining what is white in a scene.
Unlike our eyes, a camera’s digital sensor can not easily determine the type of light, since all it does is receive the light and pass it on to the camera’s computer.  It is then up to the camera to interpret the color correctly.
For that to happen the camera must be told either by an on board “auto” control sequence or “manual” (preset) input what the type of light is that is striking a subject being photographed.
The light source information allows the camera to figure out what the quality and type of light is and then establish a correct white
Once a camera knows what is white, it can determine the type of light in a scene and then accurately reproduce all other colors within the scene.

How Does White Balance Occur?
Red is around 1000K while Indigo is 10000K.
As White Balance is achieved depending on the light source, more red or green or blue and colors in between are added to achieve correct white.
Cooler colors have shorter (more energy) wavelengths while warmer colors have longer (less energy) wavelengths.
Colors can appear different depending on the pre-set established in the camera.
If the intent is to replicate the scene accurately then selecting the correct WB pre-set is important.
If there is creative intent, then an alternative setting can be selected to achieve a certain mood (more on this later).

So what happens when you change the white balance either in camera or from the RAW processing software?
The auto white balance (AWB) function analyses the composition of the scene and compares it to a set of reference metrics stored in the camera's processor and selects the set of parameters most suited to that scene type.
The outcome of this process is that the camera will roughly know what scene type it is looking at and what the colors should be. As a consequence it will shift the white point on the Planckian locus and the green – magenta proportion of all the colors to achieve a similarity between the WB corrected image and the internal reference image that it has selected as a model.
Preset white balance settings work in a similar fashion except that the scene is not analyzed, but instead the colors and the white point are shifted using predefined values of engineered algorithms for  a the specific camera and its sensor.  

Also, a ‘set white balance feature' present on a large number of cameras lets the user effortlessly create a white balance profile to match the exact lighting conditions.
What is needed is a white or neutral grey (preferably non-reflective and called usualy 18% grey) card lit by the respective illuminant that the camera while measure.
This essentially involves holding the card in front of the lens so that it covers the center of the image and pressing the set white balance or equivalent button. The camera will then analyze the image (and the color it will see will most probably not be pure grey but it will rather be a more or less saturated hue) and compensate so that after processing the patch will look grey.

         This method of achieving proper white balance yields remarkably good results due to the fact that it completely removes the influence of the colored incident light, effectively making the picture look as if the scene was illuminated by noon sunlight (white light).

Achieving White Balance can be Complicated
Achieving proper white balance in a scene in which multiple light sources of different types (color temperatures) are present can be quite tricky, such as mixed light a night, but at the same time such scenes may be quite spectacular.
Most problems with white balance appear because in many cases incident light does not come from a black body heated to incandescence (e.g. the Sun). The assortment of lighting fixtures that give off white-ish light we have today have a discrete band emission spectrum.
The incident light might also be reflected from colored surfaces. All this implies a greenish or magenta deviation from the Plankian locus.

During Post Processing White Balance can be Solved Further, if Necessary

The purpose of the tint control in most RAW converters is to enable the photographer to compensate for this deviation in addition to changing the color temperature. As it is, in- camera AWB algorithms might be very suited for some lighting situations but terrible at handling others.
NOTE:  In Lightroom or Camera RAW WB using the sliders can adjust temperature and tint.
NOTE:  As Shot White Balance for any pre-set (auto, manual or custom) is unique for each camera
NOTE:  In post processing one can add more yellow or more blue to change the temperature and/or add more read or more green to correlate tint to the temperature.  The camera is doing the same thing based on the algorithms that are associated with the sensor’s computer processor.



Examples of type of light on a scene in overcast outdoor light


Outdoor Auto WB


Outdoor Daylight WB


Outdoor Shade WB


Outdoor Cloudy WB

Examples of Type of Light on a Scene with Indoor Mixed Light


Indoor Auto WB


Indoor Daylight WB


Indoor Flash WB


Indoor Shade WB

So Why Does White Balance Matter?

Picking the right White Balance initial helps the photographer achieve a suitable reference point for realistic color in a scene.
If the white balance is not correct when the image is taken, it may cause the photographer to have to do much post processing work to get the image to look as originally intended especially since we forget quickly what our eye actually saw at a specific moment in time.
That reference is often the best and most attractive and pleasing way communicate the photographic image to an audience.
So assuming a good white balance was corrected, that it is a good reference point, a photographer can create changes in white balance to achieve a “creative” intent.
Such creative intent can, for example, suggest a specific emotion or that “feeling” that a person has as they view an image.
Photography can be photojournalistic oriented, realistic delights or an artistic expressios.  Through that entire process it is all about capturing light, displaying it in the best way;  i.e. Drawing with Light!


Creative Color Effect



Emotion and Color  figure #1




Emotion and Color figure #2


In Summary

White Balance is based on the rather complicated science of electromagnetic waves, visible light, and color.
While the human eye can adjust and view very effectively the quality, type and color of light, a camera has less ability to do this without specific instructions either built into the camera or manually given to the camera by the operator.
White balance is all about getting the best representation of color for a scene to realistically or creatively communicate photographic intent.
Thus, managing color starting with white balance in the camera is how the photographer captures light and draws with that light.


Please note this is an outline, for more information, do not hesitate to contact the author, John D. Roach at
www.jdroachphotography.com

Copyright © 2013, John D. Roach


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