ISO and White Balance

Photographers who are changing from film to digital capture will encounter a couple of new controls that handle recording functions that were formerly dictated by your choice of film: sensitivity and colour balance. One of the benefits of a digital camera is the ability to change these parameters on a shot-by-shot basis, instead of being committed to the film’s pre-set values. Fortunately, both settings are easily understood and simple to relate to similar aspects of film choice.
Sensitivity
Just as film sensitivity was dictated by the ISO rating of the film, the ISO setting on a digital camera adjusts the sensor’s sensitivity to light. The higher the ISO number, the more light-sensitive the sensor is forced to become. Most modern sensors have been designed to work best at ISO settings around 100, so the output signal from the sensor must be boosted to reach higher ISO settings.

If you envisage each photosite as a bucket that collects light, the large photosite on the left collects many more photons of light (yellow balls) than the small photosite on the right. However, the amount of random noise (grey balls) is the same for both photosites. Consequently, the signal-to-noise ratio is much better for the larger photosite – and this translates to better picture quality. (Picture supplied by Canon.)
When the signal from the sensor is boosted, any image noise (random disruption to the stability of the digital signal) in the digital signal is also increased. The resulting ‘noise’ will often be visible in your pictures. (See Larger Sensor, Better Photos for more information on image noise.)
The potential for image noise to affect picture quality is directly related to the size of the camera’s image sensor. Larger sensors with bigger photosites can collect more light and are, therefore, much less susceptible to image noise than small sensors.

The top image was photographed at ISO 100. The cropped enlargement above shows the very low noise levels typical of this sensitivity setting.

The same subject photographed at ISO 1600. Note the evidence of image noise in the cropped enlargement.
Fortunately, when you use a modern DSLR camera, noise is seldom visible unless you shoot with ISO settings above 400 – and then, it may only be seen when the image is enlarged substantially. Even shots taken at ISO 1600 can produce acceptable A3-sized prints when they have been captured with a DSLR camera. In contrast, noise is often visible at ISO 400 in shots taken with compact digicams and shots taken at ISO 1600 may be unprintable at snapshot size.
Using High ISO Settings
All modern DSLR cameras provide a range of ISO settings from ISO 200 to ISO 1600 and many extend that range down to ISO 100 and up to ISO 3200 – or higher. DSLR cameras vary widely in the range of ISO adjustments they permit. Some will only allow users to select from a pre-set range that is usually between ISO 100 and ISO 1600. Some professional DSLRs support ISO settings up to 25,600, although such high settings are only accessible through a dedicated Custom Function.
Entry-level and ‘pro-sumer’ DSLRs also include an Auto ISO setting, which allows the camera to determine the optimum ISO setting for the subject. In many cases, photographers can set an upper limit to the Auto ISO range, either in the main camera menu or as a Custom Function. Many DSLRs allow photographers to adjust ISO settings in 1/2 or 1/3 step increments. This choice is usually provided in the Custom Function menu.
In practice, you need to balance shooting convenience against potential for noise when adjusting ISO settings. Choose a low ISO for the majority of shots – including long exposures. But be prepared to increase the camera’s sensitivity when conditions demand it, taking into account the potential for noise in the resulting shot.
Be conscious of the effect temperature can have on image noise at ISO settings of 800 and above and be cautious about using the top ISO values when the temperature is above about 15 degrees Celsius. Noise can become particularly obvious when high ISO settings are combined with long exposures at temperatures of 20 degrees Celsius and above.

Use low ISO settings for the majority of shots to minimise the effect of noise. (Taken with ISO 100 sensitivity, 1/20 second at f/9.9.)

High ISO settings are useful for capturing fast action in poorly or unevenly-lit situations. (Taken with ISO 800 sensitivity, 1/800 second at f/6.3.)
White Balance
The white balance setting is used to make the colours in a digital photograph look natural under a variety of lighting conditions. It works by balancing 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.
Although you may not notice these colour casts when framing a shot, your digital camera’s sensor will record them. Fortunately, the white balance control on the camera allows you to compensate for any colour imbalances in the light that illuminates the subject.
All digital cameras include an auto white balance (AWB) setting that evaluates the light reaching the sensor and adjusts the colour balance to correct for colour shifts. The performance of these AWB controls ranges from very effective to relatively poor, with most having some difficulty removing the orange cast caused by incandescent lighting, although they usually perform well with fluorescent lighting.

The auto white balance should be able to produce a natural colour balance under a wide range of lighting conditions. (Compare the colours in this image with the colours produced by other white balance settings below.)
All DSLRs include a range of white balance pre-sets that cover common lighting conditions like incandescent (‘tungsten’), daylight, shade, cloudy, flash and fluorescent (up to three settings in some cameras).

The daylight setting adds a touch of yellow to counteract the blue of the sky.

The Cloudy setting adds yellow and red.

The Shade setting is even warmer.

The Flash setting adds slightly less yellow than the Daylight setting because the blue bias in electronic flash light is not as strong.

The Incandescent light setting is strongly blue to counteract the red/orange cast of household tungsten lighting.

The Daylight Fluorescent setting adds a hint of magenta.

The Cool White Fluorescent setting adds yellow and magenta to counteract the green cast of some fluorescent tubes.

The Warm White Fluorescent setting adds a touch of blue.
 
Many cameras also provide a manual or ‘custom’ setting, which is denoted by the following symbol: This setting 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.)
Some cameras require you to set the white balance to Custom or Manual and select the captured image. For others, this process takes place automatically. The recorded white balance data is used to correct the colour balance in the subsequent shot.
Success depends on getting the initial exposure (which captures the colour data) right. If it’s either over- or under-exposed, 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.
Most DSLR cameras allow photographers to fine-tune white balance settings along the blue/amber and magenta/green colour bands. Up to nine levels of adjustment are usually provided, enabling users to remove most residual colour casts.
Many also include white balance bracketing, which takes three shots, varying the colour tone from one to the next. Depending on the selected WB mode, the bracketing will be biased to magenta/green or blue/amber. In most cases up to three steps of adjustment in each direction is available. White balance bracketing is useful in mixed lighting when it is difficult for the camera’s auto or pre-set system to produce accurate colour reproduction.

Many DSLR cameras include controls for tuning white balance settings. Adjustments can be made on two colour axes: blue/amber and green/magenta.
In most pro-sumer and professional cameras, photographers can set white balance relative to the Kelvin colour temperature scale. This can be a huge time saver when shooting with professional lighting as all studio lights are standardised to specific Kelvin values and you can simply dial in a correction to match the lights you use. The table below shows the Kelvin temperatures for some frequently-used lighting types.

Some factors in this table deserve attention. Note that most types of lighting involve a range of Kelvin values. Only standardised lighting types (studio lights and daylight) have single values.
Note also the wider range of Kelvin values for fluorescent lighting, due to the different types of fluorescent tubes on sale: daylight, warm white and cool white being the most common. Fluorescent lighting is also ‘spiky’, with stronger emissions in specific colour bands. This can make correction of colour casts tricky, although (perhaps surprisingly) most cameras’ auto and pre-set modes come pretty close to accurate colour reproduction.
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 for these situations at higher latitudes. Consequently, some degree of fine-tuning may be required for most lighting types.
USEFUL URLs
The following websites provide additional information on the topics covered in this article.
en.wikipedia.org/wiki/Film_speed contains an interesting history of the use of ISO settings from the days of film to digital photography.
en.wikipedia.org/wiki/White_balance provides a good overview of white balance.

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