STRONG, VIVID COLOR IS OFTEN what separates a good photograph from a bad one
- 1 STRONG, VIVID COLOR IS OFTEN what separates a good photograph from a bad one
STRONG, VIVID COLOR IS OFTEN what separates a good photograph from a bad one. And being able to produce good, accurate color under any circumstances distinguishes good photographers from mediocre ones.
Modern digital cameras are designed to work under a wide variety of lighting conditions, and this includes light of a variety of colors, but in order to set the camera up to capture accurate color for different light sources, we need to know a little about the theory of color temperature of a light source.
Not all that is light, is white
Visible light, as most of us know, is a small selection of electromagnetic waves vibrating at a wavelength of between 380nm and 750nm. White light is made up of the full spectrum of these wavelengths. We know this because when we allow a beam of white light (such as from the sun) to pass through a prism, it is scattered into all the colors of the rainbow. When we look at a white object, it appears white because it reflects the full spectrum of light that shines on it.
But what if the light that falls on the subject does not contain the full spectrum? What if, for instance, the light source does not contain the longer wavelengths, and was therefore deficient in the red part of the spectrum?
The short answer is that the light will then have a slightly blue, or rather cyan quality, and the white paper will be basked in a tint of cyan. And it is not only white objects that will appear cyan, all color captured under such a light will shift equally towards cyan, and a picture taken under such conditions will have a cyan color cast, which lowers the overall color contrast of the image.
Similarly, if the light contained only those wavelengths longer than 450nm, and was therefore leaning towards the red part of the spectrum, with a deficiency in violet and blue, the light will be slightly yellow or orange, and all color photographed under such light will be tinted by the yellow or orange cast.
Often we will be blissfully unaware of the color deficiencies of the light that surrounds us. Our eyes and brains have the ability to adapt to changing light conditions, allowing us to see a white object as white, even under slightly blue or red light. Under normal circumstances this is not a problem, and can even be helpful, but when we take photographs, it can be a serious problem, because the resulting pictures will carry that color cast with it.
The theory behind color temperature
When we heat up a black metal object, it changes in color. It starts from black, then turning red, then yellow, then white, then blue.
We can measure the temperature of the black metal object when it emits light of a certain color. This temperature, measured in Kelvins, is then called the color temperature of the light that is emitted. (Kelvins, in turn is equal to the temperature in degrees Celsius + 273.15, normally rounded to 273, so 30 degrees celsius is equal to 303K)
So, put more simply, if we heat a black metal object to the point where it starts to emit yellow light, at about 3600K, the light that it emits is said to have a color temperature of 3600K. If we heat it to 5600K, it will emit light with roughly the same spectral distribution as normal daylight at noon. If we heat it even more, to about 10 000K, it will emit blue light.
We can then compare the spectral distribution of any incandescent light source (meaning it emits light through the heating of a filament), to that emitted by our original black metal object. The color temperature of the light emitted by our new light source is then equal to the temperature at which our original black metal body emitted the same spectral distribution, or color, of light.
To take a few examples, the light emitting from the flame of a candle has an orange or red tint. We would have to heat a black metal body to about 1900K (or 1627 degrees celsius, because K= Degrees Celsius + 273) before it emits light of the same color, so the light from the candle is said to have a color temperature of 1900K. Normal tungsten light bulbs, the type which are found in most homes have a color temperature of about 2800K, Tungsten studio lamps measure at about 3200K, average noon daylight is about 5500K, while flash is a little warmer at 5600K. The light emitting from a blue sky falls somewhere between 11 000K and 18 000K.
Color temperature and white balance
As we have noted above, when we are surrounded by light of a specific color temperature, our eyes will adjust to that color temperature, and we will in effect filter out the color cast created by the light source. So even if we are looking at a white object under the yellow glow of tungsten light, we will see that object as white.
Most modern digital cameras can achieve the same effect by adjusting their sensor to match the color spectrum of a specific light source.
By adjusting the camera’s white balance setting to match the color temperature of the light source, you will be able to record the colors as they would appear under white light.
Making an accurate color balance selection
Most digital cameras, whether compact or SLR, can automatically adjust the color temperature according to what it thinks is the most accurate setting, but these can be surprisingly inaccurate, even on very expensive models.
Most digital cameras will also allow the photographer to choose one of a number of preset settings, which will normally include flash, tungsten, halogen, daylight (sunny), daylight (cloudy) and daylight (shade). These are much more helpful if you know the actual type of light source you are using, but they are still only approximations. While one tungsten light may be running at 2800K, another will measure at 3200K and if you are set for the former, you may be left with a yellow cast.
The higher-end cameras will allow the photographer to adjust the white balance in steps of as small as 60K. This is highly accurate and can provide excellent results, provided you know the actual color temperature of the light source.
The preferred method, which is available in many DSLRs, is to let the camera take a color temperature reading, either directly from the light source, or from a specifically selected, neutrally colored object, such as a white sheet of paper or a grey card.
This method differs from the first mode, automatic mode, in that the camera does not take an overall reading from the scene, but rather from a specifically selected representative sample, which will allow it to obtain an accurate measurement.
For the exact methodology on how to take such a reading, refer to the camera’s manual, but it normally involves taking a picture of either the light source, or of a neutral object, such as a white sheet of paper of grey card. This exposure is then used by the camera’s internal processor to determine the actual color temperature of the light, and then adjust accordingly.
White Balance and RAW
If you are happy to shoot in raw, and are willing to go through an extra stage in post-capture, you can adjust the camera’s white balance setting after you have taken the picture on your computer. When you open the photo, either in your camera’s dedicated RAW editor, or in that of a software suite such as Adobe Camera Raw, you will be able to make a color temperature selection, allowing you to correct for any casts on screen without degrading the image.
This is a tremendous advantage, because it even allows you to try out different settings, even on those once in a lifetime shots, but I do still prefer to get it as close to perfect at the time of shooting, and then only make minor adjustments in the post capture stage.
Also bear in mind that this will only work if you shoot in RAW, not in JPEG or TIFF.
Dealing with Mixed Lighting
What if you were presented with a scene which is lit by two light sources of different temperatures, such as when you need to take a picture of an interior lit by tungsten, but with a window which shows an exterior lit by sunlight, and need both to appear correctly?
Well, in the days of film photography, there was only one real option, and that is to adjust the color temperature of the light source that you can most easily manipulate. In the case mentioned above, you would cover the tungsten light with a sheet of blue plastic to filter out the yellow rays and effectively raise the color temperature of the tungsten light to match that of the light outside.
In digital photography this approach will still work, but you also have two other methods.
First, you could take two exposures, one with the white balance set for daylight, and the other for tungsten, and then combine these in Photoshop.
Second, you could take one exposure, make sure it is in RAW, then open the file once and adjust it for daylight, and save it under a different name, then open the original RAW image again, and adjust it for tungsten, and again save it under a different name. Now open both saved images and combine them as you did in the first method.
This second method has the advantages that you do not need a tripod to keep the camera still between exposures and that you can even use it on fast moving subjects, because you do not need to identical images.
Be weary of overzealous color correction
I once witnessed, with mild horror, a photographer color correcting an image of a little girl partaking in carols by candle light. There are also urban legends of photographers taking a color reading from the sun, so as to correct for the magenta cast of the late afternoon, as it hung low in their picture of a perfect tropical beach at sunset.
There certainly are times that we want every possible color cast to be corrected, but then, there are also times when the very thing that attracted us to the photograph in the first place was the quality and the color of the lighting, in which case correcting the color cast would mean destroying the photo.
Basic Color Temperature