How to effectively use ISO and white balance

The ISO settings adjust the sensor’s sensitivity to light by tweaking the signal gain. This affects the ratio of signal (image data) to the inevitable electronic noise that is recorded with the image.

In normal daylight, most cameras can produce images with no visible noise at settings up to ISO 800 and many remain almost noise-free up to ISO 3200. Thereafter, granularity becomes increasingly visible as ISO is increased.

This photograph, taken just before sunset, shows the red/orange bias of light that is scattered by dust particles in the air. It is intensified close to sunset because the light passes through a thicker layer of air than during the middle of the day.

When there’s plenty of light, it’s easy to shoot with low ISO settings (ISO 200 in this case) and image noise is negligible.

Each photographer should determine how much noise is tolerable for individual images, depending upon how they will be viewed. Choose a low ISO whenever you can ““ particularly for long exposures. But be prepared to increase the camera’s sensitivity when conditions demand it, particularly if it’s the only way to capture the shot.

This dramatic shot of a leopard in a tree was taken after dark using the light from vehicles’ spotlights. The camera’s maximum ISO setting of 25600 was required to capture the image at the widest lens aperture.

Noise Reduction

All cameras are set up to perform noise-reduction processing on JPEG files. It may be applied automatically or at the user’s discretion. (If you shoot raw files, the only time noise-reduction processing is applied is when files are converted into editable TIFF or JPEG formats.)

Some cameras let users restrict how high sensitivity will be pushed in the auto modes. This enables you to set the ISO to Auto and give the camera more flexibility for determining aperture and shutter speed settings, while limiting the potential for unacceptable image noise.

Once you’ve gained a feel for how much noise is produced at a given ISO setting, you should be able to decide on an upper ISO limit for different situations. How much noise you’re prepared to tolerate will depend on how the image will be viewed, whether it will be cropped and how much detail you wish to preserve.

Noise reduction normally works by averaging out differences in brightness and contrast between adjacent pixels in the image. This tends to blur the image and may result in a loss of detail. Processing typically doubles the time required to record the image.

Some cameras apply aggressive noise-reduction processing by default at high ISO settings. This is undesirable as users can’t control how much blurring results. Applying noise-reduction processing when images are edited gives you better control over how much detail in the image is preserved.

The cropped image referenced earlier in this chapter, after noise-reduction processing in Photoshop. Note how the sharp edges of the pixel ‘grains’ have been blurred to make them less obvious. Many cameras apply this kind of blurring when high ISO noise reduction is selected.

Noise Characteristics

Image noise usually appears as a random granular pattern of tiny dots, often with different colours. It can resemble a ‘salt-and-pepper’ pattern with dark pixels in bright regions and bright pixels in dark regions. Image noise becomes more prevalent at high temperatures and with long exposures.

Cameras with larger sensors will always produce less noise than those with smaller sensors at the same ISO settings. For instance, the noise level produced by a Micro Four Thirds sensor at ISO 800 is roughly equivalent to that produced by a ‘full frame’ sensor (which has roughly four times the area) at ISO 3200. By comparison, a compact digicam with a 1/2.3-inch (6.17 x 4.55 mm) sensor would require ISO 100 for equivalent noise levels.

Noise becomes unacceptable when it impacts upon image quality. The point where this occurs depends on the effect the photographer wants to create and the distance from which the image is viewed. Noise is usually less obvious when images are printed than when they are displayed on a TV set or monitor screen.

Some noise may be desirable when you want to create an old-fashioned, grainy look. Some noise will be acceptable in enlargements that are viewed from a distance of a metre or more. Interestingly, some noise can actually make certain images seem a little sharper.

An example of an image captured with a high ISO setting (ISO 25600) that contains relatively high noise levels but remains acceptably sharp.

A crop from the above image, magnified roughly 3x, showing that sharpness is retained despite the increase in granularity produced by the noise in the image signal.

Different subject types can accommodate different levels of noise. For example, subjects containing large areas of even hues and tones require lower noise levels than those with plenty of inherent texture.

Colour Controls

Human eyes are good at judging what is white under different light sources. Our brains automatically correct colours, even in lighting with a strong colour cast. Unfortunately, camera sensors simply record different wavelengths of light as they are and will reflect any colour biases in the scene.

Although you may not notice colour casts when framing a shot, the camera will record them. It may apply some corrections automatically but you can’t be sure they will be adequate or result in correct colour reproduction. The white balance (WB) setting is used to make colours in a photograph look natural under different lighting conditions by rendering white objects as white in the captured image.

Auto white balance (AWB) controls are programmed to balance the colour data from the camera’s red, green and blue (RGB) channels and correct for detected colour biases. They perform well in most situations, although some types of lighting (particularly incandescent lighting) can be difficult to correct.

All CSCs come with WB pre-sets that cover common lighting conditions like incandescent (‘tungsten’), daylight, shade, cloudy, flash and fluorescent (several settings are provided in some cameras to handle different types of fluorescent lighting). Most cameras also provide one or more manual or ‘custom’ settings, which let 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.)

The recorded WB data is used to correct the colour balance in the subsequent shot. Cameras with several custom settings let you save the corrections for different types of lighting for subsequent use. So if you take a lot of shots under, say, halogen lights, you can keep a correction in the camera and simply select it when it’s needed.

Success with custom WB settings depends on getting the initial exposure (which captures the colour data) right. Most cameras provide adjustments for tweaking the colour balance along the blue/amber and magenta/green colour bands. These adjustments can be handy in mixed lighting when it is difficult for the camera’s auto or pre-set system to produce accurate colour reproduction.

Examples of the colour biases of common light sources. The top image was shot in natural daylight and has no obvious colour cast. Below it is an image shot under incandescent lights with an obvious red/orange bias. The bottom image was shot in ‘cool white’ fluorescent lighting, which adds a slight greenish cast.

WB bracketing is available in many cameras as a handy back-up when the custom setting fails to reproduce colours correctly or if you’re not sure which pre-set to use. Most cameras will record 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, with up to three steps of adjustment in each direction.

An example of mixed lighting. The lights nearest to the camera have tungsten (incandescent) globes that produce an orange-yellow bias, while the lights at the far end of the path have halogen bulbs, which have a colour output closer to normal daylight. Fluorescent lighting has been used to illuminate the rocks towards the top of the picture. It is impossible to eliminate the orange cast of the tungsten lights without changing the colour balance in the rest of the picture.

Kelvin Settings

More sophisticated cameras also provide the ability to set colour balance according 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. 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.

Note that most types of lighting involve a range of Kelvin values. There’s a particularly wide range 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.

Kelvin settings can also be helpful for tweaking colour reproduction to compensate for differences in latitude and times of the day. The colour bias of a clear blue sky or open shade can be influenced by the latitude where the photograph is taken. It’s generally much cooler in high latitudes than near the equator, as shown in the pictures on this page.

This photograph, taken in western Greenland, illustrates the cooler colour bias of high latitudes.

This photograph, taken near the equator, shows the warmer colour bias of the tropics.

Colour shifts also occur during the day, changing towards orange and red when the sun is low in the sky. Sunsets are warmer than sunrises as there is more dust in the air to scatter longer wavelengths. So some fine-tuning of WB may be required if you want to counteract these colour biases.

 

 

Excerpt from  Compact System Camera Guide.