Copyright Michael Karbo, Denmark, Europe.
Chapter 14. The white balance
When we take photographs it is both the luminosity and the colour of the light that camera has to register. An image sensor can quite easily measure the luminosity but the colours are far more difficult. Colours vary with luminosity and how can the camera choose the right colour?
Figur 48. The wall behind the lamp is white. In the photograph on the left, the camera has worked out the colour of the white itself by using the automatic program. The result is wrong because the image has a powerful orange colour cast (with a colour temperature of 4000 K). In the photograph to the right the white balance program has been set to ”Tungsten” (with a colour temperature of 2850 K), which fits in with the light given by ordinary electric bulbs. This gives a white colour, which is much more correct.
It is a matter of finding the temperature of the colour and setting up a white balance on the basis of this. A camera does this automatically but the result isn’t always good enough. If the white balance isn’t right, the image will be, for example, too orange or green – there will be a colour cast. To avoid this, many cameras give photographers the option of working out the white balance themselves, and it’s a good idea to be aware of this function.
Problems with the temperatures
We usually think of light as white, which is what it looks like. But white light is really a mixture of all the other colours, a mixture, which varies all the time. Light is completely different in the early morning than it is in the middle of the day – because there are fewer blue rays in the light.
The mixture ratio of colours is different at different times of the day and in light sources like candlelight, electric bulbs and fluorescent lighting. This means that the colours, we see as reflections, contain greatly varying qualities of colour – dependent on the source of light. Try yourself lighting up a beautiful bouquet of flowers with an energy-saving bulb or fluorescent lighting – the colours are altered totally (in this case, however, not to the better).
So we can see that nature’s colours vary quite a lot dependent on the light, which falls on them. Luckily our brains are arranged so that our sight automatically compensates for these variations. We can clearly see that a poinsettia is red whether or not it is in daylight or in an electric light. This automatic process has to be built into a camera in one way or another, so that it can reproduce colours correctly, whatever the lighting.
Purely scientifically, the light’s temperature alters according to the source of light. When there are a lot of red rays, we say that it is warm. And reversely a very blue light is cold. A camera should be set to a particular colour temperature. If it is wrong, then we will experience a colour cast in the images. This means that the photograph will have a general tone of, for example, orange or green, which doesn’t look good at all. If the damage is done, the colours might be able to be corrected with the help of an editing program.
Figur 49. Setting white balance in the menu systems of (clockwise from the top left) Canon, Fujifilm, Olympusand Nikon. The screen images look a bit confusing here but in practice, the operation is very easy. The two most important functions are probably the automatic and the user-defined white balance.
The best thing, however, is to set a camera correctly. With analog cameras, you can select a film, which made for specific colour temperatures, or you can install filters (glass plates) in front of the objective, which correct the colour temperature.
With a digital camera the adjustments are made in the image computer. This is done with the very important function, white balance. Technically speaking, it has to find the areas in the image, which are white (or grey). When these neutral coloured image areas have been located, then all the other colours can be balanced. This operation is also called grey calibration.
Popularly speaking, the white colour ”moves” dependent on the lighting’s colour quality. This is why the colour temperature should be measured before every exposure.
A camera usually has three methods, a user can choose between:
Automatic measurement works very well in modern cameras. Automatic measurement of white balance is also standard in cameras. But users can instead select one of the fixed temperatures if preferred. This is done via the camera’s menu system.
In some situations, problems can occur with an image’s colours; the lighting can cheat a camera’s automatic functions and the result is a rotten colour cast in the exposure. You can easily see this on the LCD screen, when the exposure is displayed.
This difficulty can be gotten over. You get a piece of white paper and place in the middle of the subject.
Then you select the user-designed white balance (manual preset) and press a button. The camera measures the temperature of the white surface and this gives rather a good result.
Figur 50. Here a piece of white paper has been placed in the middle of the viewer field so that a precise measurement can determine the white balance.
Processing with white balance
When a camera measures the light just before an exposure is made, it also measures the white balance (unless you have selected one of the fixed colour temperatures).
This measurement takes place with the help of the image sensor, but cameras are also found that have extra sensors, which are solely used for registering current colour temperatures.
The values are stored afterwards together with the exposure’s raw pixels. Because the correction of the lighting’s temperature itself is first made after the exposure has been taken in the camera’s image computer. Here the raw pixels are processed so that authentic colours are reproduced and an important part of this process is the balancing of white colours.
So even though the measurement of the colour temperature is taken just before the exposure is made, the image’s colours are first corrected during the following interpolation in the camera’s image computer
The measurement of the colour temperature takes place before the exposure is made. The result of the measurement is then used when the image is processed.
RAW gives the best results
It is absolutely crucial for professional photographers that the white balance is spot on; there mustn’t be the least little bit of colour cast in the exposures.
So a lot has been done for white balance in the more advanced cameras. In some models, white balance can be regulated in steps of 100 K. In others you can find white balance bracketing, where a camera makes, for example, five exposures of the same subject and the white balance is varied a little bit with every shot. The idea being that the photographer can select the best of the exposures and delete the rest of them.
But you get the absolutely best regulation of white balance with a so-called RAW-format. This is a file format, which the better cameras use to store exposures. What is brilliant about RAW files is the fact that they include the ”raw data” – i.e. the pixels, which have just come from the image sensor. This means that you can select the right white balance in your computer yourself, after having taken an exposure!
A RAW file contains information about an exposure including the colour temperature measured by the camera. So you can either select the white balance the camera has measured or correct it. This is really brilliant because you can use your own computer to change the colour temperature and see the effects on your screen.
The program Adobe Photoshop CS has a built-in RAW Plug-in, which is eminent for processing RAW files, especially the white balance.
Understanding colour temperatures
We have mentioned the term colour temperature many times. This is a colour scale in which the starting point is that colours are neutral when seen in sunlight in the middle of the day.
When the sun rises and sets colours are warmer while the light, which comes from a blue sky is colder. This fits in well with our own impressions; warm colours are yellowish/reddish, while cold colours are bluish.
If we alter the colour temperature to 17.000K, the colours become very much more realistic.
One of the big advantages of a RAW format is that the colour temperature is fixed after the exposure has been made.
The colour scale is named in Kelvin, which is a temperature scale, which resembles the ordinary Celsius scale. The starting out point is, however, different, as 0 K = -273 o C, which has no importance in this connection. Try to imagine a piece of metal, which has to be warmed up. It begins to light up with a very warm reddish colour. The hotter the metal becomes, the colder and more bluish the light appears to be.
A temperature scale has in this way been produced, which corresponds to the temperature of the light from the warmest to the coldest. The middle point on the scale is the temperature 5500 K. The colours are “neutral” just as they are in the sunlight in the middle of the day.
It is, in fact, this scale, which a camera uses to fix the white balance. The white colour is the “anchor point” in the colour scale and this is why the image’s white areas have to be identified. Then all the other colours automatically fall into place.
Diaphragm and exposure
It is most important to get the right lighting in a photograph. This can be done fully automatically because all cameras have automatic exposure programs. So anyone can take pictures without having to think much about it. But it is much more fun and much more rewarding if you have a hand it in yourself. It is quite an art juggling around with different exposure times, diaphragms, etc.
Fantastically good images in “impossible situations” can often be produced because you are familiar with your camera’s exposure meter, compensation device, etc.
Figur 51. The section in the middle is correctly exposed. The two other sections are correspondingly underexposed and overexposed (with two diaphragm steps, -2 EV and +2 EV).