White Balance

There are three light characteristics that have an importance for photography. The intensity, which sets it's brightness; The wavelength which defines the colour; and polarization which affects how light is reflected and appears in an image.

Most cameras, film or digital, can measure the intensity or amount of light in a scene using their built-in light meters. See Camera Metering Reflections in an image, which can affect how saturated colours become, can be controlled - increased or decreased - using polarizing filters.

Measuring and controlling the colour of the light recorded is slightly different and, until the advent of digital cameras, has been carried out with film cameras using a combination of the type of film, and coloured filters, called light balancing filters. Digital users have an advantage here over users of film. The colour of the light recorded is controlled automatically, all cameras having a program called AWB - Automatic White Balance. Manual control and alteration of white balance is also used, and has some advantages. Colour balance can also be altered after taking a shot on the computer using imaging software.

Here we are going to deal with the methods of adjusting and controlling the colour of light using just your camera, because getting the colours right at this stage can be better than subsequent software adjustment. Sometimes there are situations where software adjustment is still required, but any such treatment is more successful the closer the image colours are to the correct hues required.

Getting the correct colours in an image is important for there is nothing worse than taking a shot and later finding the image isn't as it seemed when you took it. Part of the problem is that our eyes, in conjunction with our brain, are very good at helping us to see what we expect to see rather than what actually exists. This is particularly true as regards viewing colours. We are very good at accepting colours as natural that later, viewed in an image, look wrong. This happens because cameras can only record what they see as it exists, not as they expect it to be.

We see images through our eyes using light. There is one constant about light, it's speed of travel, about 186,000 miles per second. The colour of light however is not constant but varies. It does not matter whether it's natural light from the Sun, or artificial light from fire, candle or light bulb. Light is heat, or radiated energy, and is emitted in wavelength form, different wavelengths producing the different colours. These range from X-rays and ultra-violet radiation at one end of the light spectrum to infrared radiation and radio waves at the other. Humans see a small middle portion of this spectrum, which is called the visible spectrum, and this is the portion from violet to red.

A lump of coal is black because it absorbs most of the light hitting it, and reflects little. Heat it, by burning it and it will glow red when hot enough, becoming a source of light - firelight - as it radiates energy. As the coal gets hotter the colour will change through orange and yellow to white, we are sure you will have heard of the term 'white hot'. Were it possible for it to get hot enough, it would eventually glow bright blue, like some stars.

As light is thus visible radiation and part of the electro-magnetic spectrum it can be measured, and this measurement is called it's colour temperature, since heat controls the colour wavelengths. Two scales are used. The most commonly known is degrees Kelvin. The other is mired [my-red]. Mired values are obtained by dividing 1million by the temperature in °Kelvin.

The earth's sole source of light is the sun. Being so far distant from the earth the light levels it produces have the same brightness everywhere on earth. Yet atmosphere and a constantly changing pattern of clouds and weather, together with daily and seasonal changes bought about by the rotation of the earth on it's axis, combine to create an infinite variety of lighting conditions. The biggest variations occur in the direction, quality and colour of sunlight and these depend on three conditions: the season, the time of day, and the weather. Further variations in the quality of natural light also occur at high altitude, and near the equator or poles.

The normal daily variation, whatever the season, is dependant on where the sun is situated in the sky. As it rises and sets it's rays of light are more oblique and have to penetrate a greater proportion of atmosphere than when it is directly overhead. In the morning, because the earth is cool, and atmospheric pollution is lower than normal, light wavelengths in the blue spectrum dominate, giving images taken at this time a cool, clean, clear look. In the evening, the sun having heated up the earth and pollution rising as a result, warmer, soft, diffused images result as hues are in the red spectrum. For the greater part of the day, between sunrise and sunset, the colour temperature is white. This colour temperature is the key measurement for photography, and is classified as mean noon sunlight with a °Kelvin of 5500, usually written as 5500K.

Other sources of light rarely produce white light. Electronic camera flash does. Its specifically designed to produce white light at 5500K. Ordinary domestic electric light doesn't, it's value varying depending on whether its Tungsten [bulb] or Fluorescent, and is often in the range 2500K -3500K, giving a yellow to pale straw hue. Candle flame is around 2000k and gives an orange/yellow hue. Studio lights come in two varieties. Flash, which is often adjustable in it's range from 5500K to 3200K, and photoflood, which is Tungsten at 3200K. Then there's Fluorescent, Sodium Vapour, Halogen etc, all of which can give off different hues.

The first method is carried out without the knowledge of the majority of film camera users. Most film is, as with electronic flash, set to produced the correct colours in an image at 5500K . This is known as Daylight Balanced. As most users only use their camera either in the day, or indoors using flash, colour balancing is taken care of. Problems only occur when shots are taken outside these parameters.

There is also film produced that is known as Tungsten or Type B film, because it's specially balanced to suit shots taken under Tungsten lighting at 3200K without flash and produce the correct colours. It prevents the warm yellow hue that would occur if ordinary daylight balanced film was used.

The drawback with this method is that outside of their correct colour temperature ranges, and there is some degree of latitude, they produce the wrong colours. Use Tungsten balanced film in daylight, or with flash, and the shots will exhibit a strong cool blue hue, known as a 'colour cast'. Use daylight film under domestic tungsten lighting, as most do, and unless flash is used a warm cream/yellow/brown cast ensues, resulting in unpleasant skin tones, a pallid yellow jaundiced look sometimes.

Sometimes pleasant shots can result. Many photographers like to take shots at or near sunset. Shots of the setting sun can and often do produce spectacular images, aided by the warmth of the reddish hues, resulting from the use of daylight film. Shots taken outdoors at night can also produce a variety of colours, the different lighting giving off different casts, resulting in some interesting images, especially of buildings.

Another method much favoured by photographers is to use light balancing filters. These are fitted in front of the camera lens and thus alter the light hues entering it. There are many types of filter used in photography. Probably the best known single filter of any type is the UV. The Ultra-Violet filter is regarded by many as a universal standard fitment for SLR lenses. As well as offsetting the effects UV light has on an image - distant views can have a blue cast and look hazy - these colourless filters also protect the lens front element from scratches and dirt. Unlike most other filters they don't stop any visible light from entering the lens.

The most common light balancing filters are either bluish or yellow-orange, come in different strengths, and are often called 'cool down' or 'warm up' filters. As well as being used to correct the colour casts most often encountered these filters are also used to generate them. Landscape photographers often like to use a pale straw 'warm-up' filter to add or increase the red hues in an image. If you are using a type B film outdoors, adding a warm-up helps to offset the very cool cast that would otherwise result. Cool down filters are used indoors with daylight film and offset the yellow cast.

A problem with most filters is that they reduce the amount of visible light entering the lens, resulting in slower shutter speeds or wider apertures. How much light they stop depends on their strength, and varies from as little as 1/3 stop to as much as 3 stops. So using them sometimes requires good light levels, a tripod, or high speed film

Users of digital cameras have an advantage over those using film. This is the ability to control colour temperature in an image without the need for filters. All digital cameras use the method called AWB - Automatic White Balance - to control colour balance automatically. As the image is processed after taking, a point in the image is taken as the 'white point' - 5500K - around which the other colours are balanced accordingly. This is usually pretty reliable within a fairly wide range, but as with daylight balanced film, it can run into problems at either end of the Kelvin scale, and particularly the lower, warmer end.

To help, many cameras have an additional range of pre-set white balance options that can be set if the AWB is not producing the colour balance of your liking. These are often similar to the picture program modes in that they are intended to match a particular type of weather or location. Clear Sky, Shade, Cloudy, Fluorescent, Tungsten, Flash, Etc. Some cameras use the Kelvin scale instead, but this relies on the user being familiar with it, and knowing what level to set.

Many cameras also offer the ability to set a custom white balance. What is a custom white balance? Well, you may encounter lighting conditions neither the AWB or pre-sets can cope with, say a mixture of lighting, natural and artificial, or different types of artificial. Setting a custom white balance allows you to overcome, or at least reduce, any colour casts you may have problems with, even if you cannot eliminate them altogether.

Quite apart from getting the correct colours in an image, there are other uses for pre-set WB and custom WB settings. You can 'misuse' pre-set WB's to warm up and cool down a scene, or make tailor made custom WB's to do the same thing. The advantage with this is no light stopping filters are needed when the shot is taken.

Although the specific method of setting a custom white balance varies between cameras, the general methods used apply regardless, and this is what we will deal with here. The basics of setting a white balance, setting one to use with filters, and how to set one that can warm up or cool down a specific scene without having to use a filter when you take the shot.

We know AWB works by setting the white point in an image that it relates all the other hues to. It shifts whatever hues exist to bring them into line with the Kelvin scale. What this means in very crude practical terms is that it adds reddish hues to a cool scene and blue hues to a warm one, in much the same way that photographers use light balancing filters.

However there are occasions when AWB doesn't work quite as well as it should. It is also the case that some cameras have AWB that works much better than others. So apart from AWB most digital cameras offer a range of pre-set WB's to cover a range of lighting situations you may encounter. Using a pre-set WB is really just a question of deciding what the weather is like and setting a WB to suit. And of course one of the benefits of digital immediately comes to the fore, in that you can soon see, by viewing the shot after taking, or pre-viewing it if you use a digicam, as to whether the colours are correct and try another setting if they are not, or for comparison.

We would say that how often you need to override AWB very much depends on the camera you use. Getting to know your camera and using it to take plenty of test shots is invaluable and will soon show where it has any failings. You may well find that it is only in exceptional circumstances that the need to override the AWB occurs. On the other hand it may produce unacceptable colours outside a very narrow range.

To show the practical problems we made up a simple colour check chart on the computer in Photoshop using a selection of colours and printed it out on heavyweight matt paper.

The original image file is reproduced as fig 1. In fig's 2 & 3 are shots taken of the chart in daylight using two different digital cameras. The default camera settings, where there was a choice, were used. As you can see the colour reproduction from both is quite close although the digicam did not produce a pure white. The DSLR produced a closer image as far as saturation goes and a pure white. Neither produced high contrast images. It's interesting to reflect on the results because they can tell you quite a bit about how a camera will deal with different lighting conditions and how they have been programmed to react. As far as these cameras go the tests serve to confirm their reactions.

The 3mp digicam, one of the first ever made, produces quite un-saturated low contrast images and almost never burns out highlights. It was made and sold specifically to work this way, to suit those working in the medical and forensic science areas, the police etc, indeed anyone who needs to ensure that images produce the greatest range of detail above all else, and the only downside is that some of the time, even in bright sunlight, images can look dull, i.e. low contrast, as the chart image shows. The 6mp DSLR on the other hand does burn out highlights quite easily despite producing fairly accurate colours without high contrast. However neither camera really produces good representations of either magenta or cyan.

As we stated earlier most problems with AWB occur with shots taken under artificial lighting, and usually indoors. So here is the original chart image again, Fig 3, with two more images from the digital cameras, Fig's 4 & 5. This time the shots of the chart were taken under tungsten lighting. In other words ordinary household bulbs.

As you can see some odd but not entirely unexpected results have been obtained. The digicam's AWB seems to have given the image a 'pinky' look whilst the DSLR's is very 'brown'. Getting these kind of results is quite common.

The answer is to try a pre-set white balance or make a custom one to suit the lighting. A problem here is that some cameras don't have a custom white balance option whilst others don't have pre-sets. Others only have AWB. If you have a AWB only camera the only option open to you is to alter the white balance in software. Most times this can work quite well, but not always. The problem is that AWB can often skew the colours in trying to obtain the correct balance, so trying to correct the colours later may result in one being corrected but putting another out. If your camera has a pre-set to match the lighting, try it. You might find it works well.

However the best option, and the one which returns the most consistent results, is to set a custom white balance. So how to you set one? Apart from knowing how to set one on your particular camera, which will be in the instruction book that came with it, there are some other points to note;

If you are familiar with or have seen the use of a hand held light meter you will understand that the same general principles apply.

These requirements might seem obvious to those who have a lot of photographic experience, but there are many that have confused what an 18% grey card is, and what it is used for, and have thought that it can be used to set a white balance. For those that do not know, an 18% grey card, which can be obtained from most good camera shops, is used to set an exposure meter reading in difficult lighting conditions. You actually use it in much the same way that you use white paper to set a custom white balance, except that the cameras exposure meter takes it's reading from the card. This is why the confusion has undoubtedly arisen. Why 18% grey? Well that's the mean average light reflectance percentage from a scene.

The idea of a custom white balance is that you are telling you camera. Here is a colour, use this colour as the base white point around which you set all the colours in an image. Now it follows that if you don't use white as the basis on which the camera makes it's calculations, the colours will not be as they should, they will be wrong.

Here are some more colour chart comparisons. Again Fig 6 is the original chart. Fig's 7 & 8 are from the cameras using a custom white balance setting from the tungsten lighting under which the previous shots, fig's 4 & 5, were taken.

The results are much more in keeping with both the original file, and those produced under daylight, although they are by no means perfect. Once again the DSLR has produced more saturated colours.

Using pre-set WB's and making custom WB's to warm up and cool down an image is easy to do. If you decide that you need to do this with an image on the spur of the moment you require access to a range of compatible pre-sets, so lets deal with this first.

If you are used to using daylight balanced film and know and like the results you get under the different lighting conditions it's easy to replicate this simply by using a pre-set that sets the WB to around the 5500K mark. 'Shade' we have found is probably the closest that you can get if your camera has 'picture' pre-sets rather than a Kelvin scale choice. Under normal conditions this will give cool images in the morning, and warm in the evening.

If you are shooting in strong evening sunlight and want to eliminate, or reduce, the yellow/orange hues try using the tungsten pre-set. This will cool the colours down, just like it's meant to do indoors. But it may be too strong. To increase the colours try the 'Shade' pre-set which will warm them up. There is no hard and fast rule here. Each and every cameras pre-sets will work slightly differently and it really is a question of trial and error. And the results obtained will alter as the light does, which is no different to using light balancing filters.

If these options aren't suitable enough for what you want then another option is to make some custom white balance settings you can try. A lot of cameras have more than one custom white balance setting option and this can be very handy as you can make some WB's for use when the situation arises.

To make warm up and cool down balance options all you need is either some Red and Blue paper, or filters of the same colours and white paper, either will do. To start with we would recommend that you make just two WB's, one warm up and one cool down, so as not to cause confusion. You can adapt them later, making them stronger or weaker, as the need arises.

To make the WB's simply use the red and blue paper to set a custom white balance just as you would using white paper. If your using filters instead just fit them on the front of the lens as normal before setting the white balance using white paper. Obviously you must make these custom settings under normal daylight conditions.

However there is one little twist here. When you use these custom white balances you use them the opposite way around. Normally you would use a red filter to warm up a scene, and a blue one to cool it down. Here you use the custom balance set using red to cool down an image, and the one set using blue to warm it up. Confused? To help you here are a selection of custom white balances we made, the colours we used, and the result when used to shoot the test chart under daylight conditions.

There are four colours. Red. Light Red/Pink. Light Blue. Dark Blue. You can see the effects the different colour strengths have on the chart colours. We have also used a 18% grey card setting just to show the effect. Unlike the other WB's this doesn't alter the colour hues much, just makes them darker.

	strong blue cast added. offsets warm or 'brown' images
	weak blue cast added. a lesser effect to above
	strong red cast added. Gives very 'mellow' images
	strong red cast added. Gives very 'mellow' images
	no cast added, just darker. Increases saturation.

Often, using white balance settings is a matter of trial and error. Most times AWB works quite well. But when it doesn't, we hope what we have written here will be of some help, and a starting point if nothing else, in the quest for the colours in an image that you want.