Saturday, November 6, 2010

White balance (WB)

What is WB?
Our eyes and brain make a wonderful partnership. Together, they automatically adjust the exposure of whatever it is we are looking at to achieve a balanced image. More importantly, they correct any colour cast so that anything white actually looks white, regardless of the ambient light.

Unfortunately, digital cameras are not so clever. They record the scene as they see it − within their limitations. This is why, in tricky lighting conditions, we have to set the white balance to ensure the result we want. It is also why, to get results of the optimum colour, we usually have to tell the camera the colour temperature of the light falling on to the scene.

In simple terms, light is made up of the three primary colours − red, green and blue. These colours are present in different proportions in all light sources. For example, tungsten lights have more red in them than fluorescent lights, which are greener. This proportion of colours is measured as the colour temperature.

If you are shooting your images in any mode other than RAW, the camera will post-process the image to make the colours in the scene as accurate as possible. However, this is not always as easy as it seems − the colour temperature of the light falling onto the scene affects the way the camera sees the colours and, unlike our brains, it does not automatically correct it. For example, with no correction, a white wall photographed under tungsten lighting will appear very yellow, and under a fluorescent light will look very green. This is why all digital EOS cameras have the ability to set the white balance to suit the ambient light, no matter how complex the lighting conditions are.

What do the settings mean?
There are seven standard white balance settings, your choice of which will depend on the type of light you are shooting in.

Auto White Balance
You can use this setting as a default in most straightforward lighting conditions. Auto White Balance works by evaluating the scene and deciding the most appropriate white point in it. The setting works reasonably well if the colour temperature of the ambient light is between 3,000-7,000K. However, if there is an abundance of one colour in the image, or if there is no actual white for the meter to use as a reference, the system can be fooled, resulting in an image with a colour cast.

Daylight setting
Use this setting if you are shooting in bright sunshine. It will balance for a colour temperature of around 5,200K, which is actually very slightly cooler than noon sunlight. However, it is very rare that you will actually be shooting at noon and so this setting will work best for the greatest part of the day.

Shaded places
Although we perceive shaded areas to be colder, the colour temperature is actually higher (bluer), usually around 7,000K. This setting is most suited to areas of light shade rather than very heavy shadow.

Cloudy or hazy
This sets a colour temperature of around 6,000K. It is best used on days when the sun is behind the clouds, creating a very even and diffuse light.

Tungsten setting
The first of the artificial lighting settings, this assumes a colour temperature of around 3,200K and is suitable for most tungsten lamps that normally emit a yellow light. It is the equivalent of an 82-series blue filter used with a film camera to correct for the same colour cast.

Fluorescent setting
The second artificial light setting is set for around 4000K, the approximate colour temperature of fluorescent lights. The problem with fluorescent lights is that there are six types, each with a different colour temperature. They also emit an interrupted spectrum with peaks over quite a wide range. To complicate things further, they also change over time, gradually altering the colour temperature of light they emit. This setting has the same effect as fitting an FL-D filter to a film camera.

Flash setting
For use with either a built-in flash or an external Speedlite. Flash is a very white light with a colour temperature around 6,000K.

All these settings still rely on the camera doing some calculations to obtain the correct colour balance. However, there are two further settings which give you total control.

Custom white balance
This allows you to tell the camera which area in the scene is supposed to be white. The camera can calculate the colour shift required to make that surface white. It then applies that shift to all colours in the scene to provide a correct colour balance to the image, whatever the lighting.

This enables you to set the colour temperature in degrees Kelvin in 100K increments from 2,500 to 10,000K depending on model. If you have a separate colour temperature meter then this may be the best setting to use as you can set the exact colour temperature shift needed. But, remember, if you do this you will need to take a few test shots to calibrate your colour temperature meter with the camera’s meter.

PC-1, PC-2, PC-3
In addition, on professional EOS digital cameras there are three custom white balance settings − PC-1, PC-2 and PC-3. These allow you to save the three white balance settings that you regularly use. This is useful if you do a lot of studio photography, for example, and always use the same lights. It enables you to save the colour temperature of those lights so you do not have to colour balance each time. However, initially the settings need to be made on the camera via a computer using supplied software.

White balance bracketing
If you find that you still cannot get the perfect colour balance, then EOS digital cameras from the 10D onwards have a white balance auto bracketing function. This allows you to bracket the white balance setting in the same way that you can bracket exposures. You can select the level of change between the images up to ±3 steps in full-step increments. The images are then recorded in the sequence: 1 - set colour temperature, 2 - bluer colour, 3 - redder colour.

With all these options, it is possible to obtain a completely neutral tone in most shooting situations. However, is this always best? Consider a fairground where there is a diverse mix of light sources − tungsten giving a yellow glow, fluorescent adding some green, not to mention all the neon lights. If you were to balance all the light sources present, the result could end up looking very clinical and fail to convey the fun, warmth and atmosphere of the show.

So do not always assume neutral is best − be a little creative and see what happens.

Working in RAW
If you shoot in RAW, white balance will not affect you at the time the exposure is made. White balance corrections are normally applied by the camera in post-processing of the image, before it is saved to the card as a JPEG file.

By shooting RAW, you avoid this processing as the image saved on the card is exactly as captured by the CMOS sensor. It is then up to you to adjust the white balance in your RAW file editing program.

One of the advantages of shooting RAW files is that you can apply different white balance settings to the image to see which give the most natural, or most attractive, results. The original RAW file remains unchanged. All the work is done on copies of the RAW file, which means that you can return to the RAW file and try again if the initial results are not what you want.

Setting the white balance
It is possible to set auto white balance for all your shots and let the camera sort out the light, or to select the white balance symbol appropriate to the lighting conditions. However, no matter how good these settings are, they will never produce the perfect white balance in all situations.

Instead, use the following procedure and you will end up with images that are properly white balance. Unless you are working indoors light changes constantly, so you will need to repeat the procedure for each new scene.

You need a sheet of white paper. With your scene set and the lighting arranged, place the card in the scene. Making sure that the white card covers the centre circle marked in the viewfinder, take a shot. The autofocus may have trouble focusing on the flat card, so focus on the edge of the card and then recompose.

Find the menu item ‘Custom WB’ and select it so that the custom white balance screen appears. Now turn to the image shot in the previous step and select this. The white balance data from the image will be imported.

After exiting the menu, select custom white balance from the white balance settings. The pictures you shoot will now be balanced to your test image.

Colour temperature
It is useful to know where the term ‘colour temperature’ comes from and what it means.

Colour temperature is measured in degrees Kelvin (K). The Scottish mathematician and physicist William Kelvin proposed the absolute, or Kelvin, scale in 1848. This scale uses −273.15°C as its zero point or ‘absolute zero’. The colour temperature is simply the colour a standard black body (a piece of metal which is perfectly black and reflects no light) glows at a certain heat. Surprisingly, the cool colours are red and orange, around 2,000-3,000K, while the warm colours are the blues at the 20,000K end of the spectrum. Neutral white light is 6,504K.

Comparison shots
The following pictures of a stamen against the white petals of a lily flower were all taken in daylight in the middle of the day. Each was taken at different white balance settings. The colour changes to the images show, in effect, the ‘filtration’ added by the camera to balance with the different light sources.

Auto − this close-up of a lily stamen was taken in midday window light so the Auto setting has actually done quite a good job of making the lily’s white petals white although there is a hint or red.
Daylight − this setting has done slightly better, with no red and the white being a cleaner white.
Shade − this setting has also achieved a reasonable colour balance, however, you can see it has added yellow to compensate for blue that would normally exist in a shady scene.
Cloudy − as with the shade setting, the cloudy setting has achieved a satisfactory result, but again there is still a hint of yellow in the whites.
Tungsten − here the camera has added blue to compensate for the yellow that would be present under tungsten lighting giving the whites a very blue appearance.
Fluorescent − this has had a similar effect to the tungsten setting, but the camera has added the equivalent of a magenta filter to compensate for the green emitted by fluorescent lights.
Flash − this setting has produced a result very similar to the shade setting as they set a very similar Kelvin temperature. If anything, the flash setting has done slightly better by adding less yellow to the image.
Custom − here we set the colour balance manually by using a grey card. This has produced a result with clean, natural whites.

No comments:

Post a Comment