The white balance setting is used to make the colours in a digital photograph look natural under different lighting conditions. It works by adjusting the colour balance so that objects which appear white in reality are rendered white in the image. To do this, the camera has to evaluate the ‘colour temperature’ of the light illuminating the subject and correct for its relative warmth or coolness of white light.
Human eyes are very good at judging what is white under different light sources and our brains apply automatic correction to produce a colour-correct impression. Digital cameras have to rely on auto white balance (AWB) adjustments, which can often fail “” and may even introduce unwanted blue, orange or green colour casts.
AWB adjustment balances the colour data from the camera’s red, green and blue (RGB) channels. These primary colours are found in all light sources in varying proportions, depending on the colour temperature of the light source. With a high colour temperature, the light has more blue; with a low colour temperature it’s redder.
Different types of lighting have different colour temperatures, which allows camera manufacturers to create a range of pre-set colour balances that should, in theory, counteract the colour cast and restore cast-free colours.The illustrations on this page show the effects of the colour corrections that typical presets apply for different types of lighting.
Typical colour corrections applied by white balance presets. Top row from left: Auto white balance should reproduce natural colours with no obvious bias; Daylight correction adds a touch of orange to counteract the blue from cloudless skies; Shade correction intensifies the orange correction. Middle row from left: Flash correction adds a hint of yellow to remove a slight blue bias in electronic flash light; Daylight Fluorescent lamps require a fairly strong orange-red correction to counteract strong blues in their emission; Cool White Fluorescent lighting needs a slight magenta bias to remove greenish casts. Bottom row from left: Warm White Fluorescent light is biased towards red and orange and needs a blue filter to correct it; Incandescent lighting is very orange and is corrected by a strong blue filter; Cloudy skies need a hint of yellow to warm a slight blue cast.
In most DSLRs, photographers can set white balance relative to the Kelvin colour temperature scale. This can save time when shooting with standardised lighting as you can simply dial in a correction to match the lights you use. The table on the following page shows the Kelvin temperatures for some frequently-used lighting types.
Most types of natural lighting (and some artificial light sources) involve a range of Kelvin values, which means the actual colour may fall anywhere within the specified range. Some types of artificial lighting, notably fluorescent lighting, are ‘spiky’, with stronger emissions at specific colours.
The Kelvin values for fluorescent lighting vary according to the different types of fluorescent tubes on sale: daylight, warm white and cool white being the most common. This can make correction of colour casts tricky.
The colour bias of a clear blue sky or open shade can be influenced by the latitude where the photograph is taken. Kelvin values are usually higher in higher latitudes and some degree of fine-tuning may be required.
On-screen adjustments of colours along Blue/Amber and Magenta/Green axes are available in all DSLR cameras for fine-tuning colour balance settings.
To overcome problems associated with varying colour balances, all DSLRs provide a manual or ‘custom’ setting, which lets you measure the colour of the illuminating light and use the result to remove unwanted colour casts. The process is straightforward. Simply cover the subject with a plain white object (sheet of paper or white card) and set the lens focus to manual before taking an exposure or white balance reading to capture the colour of the illuminating light. (Some cameras can record the light without taking the shot.)
Success depends on the initial exposure, which captures the colour data. If it’s not correct, subsequent shots may be off-colour. Some cameras alert photographers when this happens ““ but most don’t so you should always check your shots when using this strategy.
Shooting raw files is the best way to deal with white balance in difficult lighting because you can adjust white balance AFTER shots are taken. Raw files also provide a broader range of colour temperature adjustments, which may been needed to cope with subjects containing a mixture of different lighting types (fluorescent, halogen and incandescent, for example).
Adjusting white balance in a raw file is quick and easy. You can either tweak the temperature and green-magenta sliders until colour casts are removed, or simply click on a neutral colour (white or grey) within the image. This reference can be used to quickly correct other shots taken under the same lighting.
Image Tone Adjustments
Most DSLRs include special image tone adjustments (or ‘picture styles’) for imparting particular ‘looks’ to photos. As a rule, these controls can only be used in the P, A, S and M shooting modes and only with JPEG files.
It’s common to have a Standard default setting plus presets for Vivid, Neutral, Portrait, Landscape and Monochrome. Most cameras allow users to adjust the sharpness, contrast, colour saturation and colour ‘tone’ (or hue) parameters within each setting.
Left: The Standard, Neutral and Faithful image tone adjustments all produce natural looking colours.
Right: The Monochrome setting converts the colour image into black and white.
Sharpness adjustments work mainly on edges and allow photographers to sharpen or ‘soften’ their shots. Contrast and saturation (colour vividness) adjustments are similar, with the minus settings reducing and the plus settings increasing the selected parameter.
Picture style adjustments can often be applied post-capture when converting raw files into editable formats. Some cameras also allow them to be applied in-camera (to JPEGs) and a few allow users to save adjusted settings as new picture styles for future use.
Digital Filter Effects
Digital filter effects have become popular in recent times, particularly in entry-level cameras. They’re usually accessed through the main menu and typically include Toy Camera, Retro, Miniature, Reversal Film, Monochrome and Bleach Bypass settings. These effects can only be applied to JPEGs and some are quite extreme. Different manufacturers provide different levels of adjustment for these effects.
Examples of some of the digital filter effects provided in many modern DSLR cameras. Top row: the unedited image, Toy Camera, Grainy B&W; bottom row: Reversal Film, Miniature, Bleach Bypass.
The main problem with all of these adjustments is that the adjusted settings are locked into the image file. The adjustment range is also somewhat limited, compared with the adjustments available in image editing software ““ including raw file converters. We suggest that you use them sparingly.
Make sure you keep an unadjusted copy of the shot by shooting RAW+JPEG pairs. Then, if you don’t like effect, it’s easy to go back to the unadjusted image and start again.
[This article is an excerpt from Digital SLR Pocket Guide 3rd Edn]
