how white is white … Part 1

How to balance your whites

Some times when I’m looking at my RAW data the Images just don’t look quite right. They may look slightly blue or yellow, the problem may be that the white balance is off. White balance (WB) correction is the process of removing “unrealistic colour casts”, so that objects which appear white in person are rendered white in your photo.  Proper camera white balance has to take into account the “colour temperature” of a light source, which refers to the relative warmth or coolness of white light.  Our eyes are very good at judging what  white looks like under different light conditions and will automatically adjust it, however DSLR’s often have problems with auto white balance (AWB).  An incorrect WB can create unsightly blue, orange, or even green colour casts, which are unrealistic and particularly damaging to portraits (unless thats a look you are looking for in the final image).  Performing WB in traditional film photography requires attaching a different cast-removing filter for each lighting condition, in digital this is no longer required but you can still do it if you want.  Understanding digital white balance can help you avoid colour casts created by your camera’s AWB, thereby improving your photos under a wider range of lighting conditions.

In short

A surface reflects light. If the light that hits that surface isn’t “pure white” it will affect the reflected colour of the subject. e.g. Light from the sunset is a different “colour” to the light from a florescent lamp.

Colour temp

Colour temperature describes the spectrum of light which is radiated from a “blackbody” with that surface temperature.  A blackbody is an object which absorbs all incident light– neither reflecting it nor allowing it to pass through.  A rough analogue of blackbody radiation in our day to day experience might be in heating a metal or stone: these are said to become “red hot” when they attain one temperature, and then “white hot” for even higher temperatures.  Similarly, blackbodies at different temperatures also have varying colour temperatures of “white light.”  Despite its name, light which may appear white does not necessarily contain an even distribution of colours across the visible spectrum:

Note that 5000 K produces roughly neutral light, whereas 3000 K and 9000 K produce light spectrum which shift to contain more orange and blue wavelengths, respectively.  As the colour temperature rises, the colour distribution becomes cooler.  This may not seem intuitive, but results from the fact that shorter wavelengths contain light of higher energy.

Why is colour temperature a useful description of light for photographers, if they never deal with true blackbodies?

Fortunately, light sources such as daylight and tungsten bulbs closely mimic the distribution of light created by blackbodies, although others such as fluorescent and most commercial lighting depart from blackbodies significantly.  Since photographers never use the term colour temperature to refer to a true blackbody light source, the term is implied to be a “correlated colour temperature” with a similarly coloured blackbody.  The following table is a rule-of-thumb guide to the correlated colour temperature of some common light sources:

Color Temperature Light Source
1000-2000 K Candlelight
2500-3500 K Tungsten Bulb
3000-4000 K Sunrise/Sunset
4000-5000 K Fluorescent Lamps
5000-5500 K Electronic Flash
5000-6500 K Daylight with Clear Sky
6500-8000 K Moderately Overcast Sky
9000-10000 K Shade or Heavily Overcast Sky

Since some light sources do not resemble blackbody radiators, white balance uses a second variable in addition to colour temperature: the green-magenta shift. Adjusting the green-magenta shift is often unnecessary under ordinary daylight, however fluorescent and other artificial lighting may require significant green-magenta adjustments to the WB. Fortunately, most digital cameras contain a variety of pre-set white balances, so you do not have to deal with colour temperature and green-magenta shift during the critical shot. 

SonyA100_whitebalance_menu

RAW

The best white balance solution is to photograph using  RAW it allow you to set the WB *after* the photo has been taken.  RAW files also allow one to set the WB based on a broader range of colour temperature and green-magenta shifts.

Performing a white balance with a raw file is quick and easy.  You can either adjust the temperature and green-magenta sliders until colour casts are removed, or you can simply click on a neutral reference within the image e.g. the WB dropper in Lightroom.  Even if only one of your photos contains a neutral reference, you can click on it and then use the resulting WB settings for the remainder of your photos (assuming the same lighting).

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About shotbyscott

Just another photography blog.. I also like all kinds of accessories
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