Nikon Owner Issue 18
Balancing Act
Setting an accurate white balance value does not have to be a matter of guesswork; thanks to the ExpoDisc tool it really is very straightforward. Simon Stafford explains.
Regular readers of my articles and books about photography will no doubt be familiar with my opinion on the importance of adopting a disciplined camera technique to ensure you get as much right in camera at the time of the original exposure, rather than relying on remedial actions with a computer at a subsequent stage.
All photographers shooting with digital cameras strive to achieve an accurate exposure of their subject but I wonder how many make the same effort to set an appropriate white balance value? I suspect most are content to rely on their camera's automation to sort out white balance - and why not? - it is a valid function designed to assist the photographer. The complex algorithms developed by manufacturers and in-camera signal processing performed by the current crop of digital cameras mean that they are all capable of excellent results across a wide variety of colour temperature values. So why do the same manufacturers build in alternative methods of selecting and applying a white balance value to your pictures? The answer is simple; there are compelling reasons for avoiding your camera's automatic white balance feature - that's why!
I suggest, strongly, that selecting your camera's pre-set (custom) white balance feature offers several advantages for the following reasons:
Generally, the range of white balance values available via the automatic white balance feature is narrower than if you select a white balance manually.
Altering the composition of a scene by, for example, shifting the zoom ring on a lens can induce a change in the white balance value set by the automatic white balance feature, because the camera may 'see' a different ratio of colours reflected from the scene. The same problem can occur when you want to shoot a series of pictures with a view to 'stitching' them together to create a panoramic view. Altering the composition for each shot can result in them having a different white balance value, which will spoil the continuity of the panorama.
Setting a specific white balance value allows you to 'clean-up' any overall colour cast that may affect the scene you are shooting, which is essential when you shoot in the JPEG or TIFF file format because trying to remove colour anomalies in these files at a later stage can be fraught with difficulty.
Manual selection of a white balance results in predictable colour, and therefore repeatable results. This is analogous to choosing a particular film because you know how it will react in any given circumstances.
Selecting a specific white balance value means consistent colours, which aids the efficiency of your workflow when it comes to processing your images. This is especially important if you shoot in the RAW file format, as these will almost invariably require some degree of adjustment in your computer, and getting colour right in the camera means you have one less image attribute to worry about.
As an example of the different range of white balance values that cameras offer consider the Nikon D70s and D2X; the automatic white balance feature of both cameras extends from 3500K to 8000K, whereas setting it manually on the D70s provides values from 2700K to 9200K, and 2500K to 10,000K with the D2X. That extended range for the manually selected values is significant, especially in the case of the lower colour temperatures.
In light, with a low colour temperature say around 4000K, or less, I have found the automatic white balance function of Nikon D-SLR cameras tends to set a colour temperature (Kelvin value) that is too high, which is probably due in part to the very low level of blue wavelength light present in such conditions. This becomes an important issue if you select the Auto white balance control, because the photosites on the camera's sensor that detect light in the blue wavelength range receive so little information. Consequently, left to make its own decisions the Auto white balance control will, typically, render a picture that appears to be overly warm (red/yellow). Hence Nikon suggest that for practical purposes the lowest colour temperature the Auto white balance can deal with is 3500K. However, the light from many incandescent light sources is, generally, in the range of 3000K to 3500K, for example a 100W domestic light bulb emits light with a colour temperature of about 2900K. Hopefully, this explains the first reason I give at the beginning of this article.
At this point it is also worth noting that Nikon's interpretation of colour temperature for some of the pre-determined values available on their digital cameras is a little curious. For example, the white balance value of 5200K for the Direct Sun setting, and 5400K for the Flash setting is rather on the low side. I say this because daylight film is usually balanced to around 5500K (see below), and most Nikon Speedlights emit light with a colour temperature between 5800K and 6000K. Hence, the problem is reversed in these instances and pictures can look too cool (blue), because in sunny conditions the ambient light will frequently have a colour temperature that is appreciably higher than 5200K; likewise light from a Speedlight flash will also have a colour temperature above 5400K. So, there are another two good reasons for using the pre-set (custom) white balance option!
Colour Temperature
The colour of light is often referred to as its 'colour temperature', which is expressed using the absolute centigrade scale in units of degrees Kelvin (K). It sounds counter-intuitive but warm light (higher red wavelength content) has a low temperature and cool light (higher blue wavelength content) has a high temperature.
Why is this? Well, the colour temperature of a light source correlates to the colour of a 'black body radiator' (a theoretical object that re-emits 100% of the energy it absorbs) as it is heated; its colour changes from black, to red, orange, yellow, through to blue, as it gets hotter. The spectral output of a particular light source is said to approximate to a 'black body' at the same temperature, thus at low colour temperatures, light contains a high proportion of red wavelengths, and conversely at a high colour temperature it comprises, predominantly, of blue wavelengths.
Generally, during its manufacture most film is balanced to either daylight under a clear sky at mid-day (5500K), or the light emitted by a tungsten photoflood lamp (3400K). If the temperature of the ambient light you are shooting under differs from these values your photographs will take on a colour cast, which you have to counter by using colour correction filters.
Digital cameras are far more versatile and most allow you to set a specific colour temperature (white balance) in addition to their automatic white balance capabilities, so when you view the image in the camera, or computer, the colours are matched to your chosen white balance value. Assuming this value corresponds to the colour temperature of the prevailing light the scene will be rendered without any colour casts. Of course you can use the white balance feature creatively by setting an alternative value, which does not correspond to the prevailing light and thereby induce a colour shift deliberately.