August 18 2016
Light is not always white, even when it appears that way to the human eye. Our brains adjust automatically to slightly different coloured light and see the scene as if it is lit by white light. Automatic White Balance (AWB) system in digital cameras or adjusting in Adobe Lightroom, makes it easy to get true colours under different lighting conditions.
Colour is not what it seems
Each light source is a slightly different colour. The measurement scale used to denote the colours is called the colour temperature, expressed in degrees Kelvin (K). This temperature refers to the theoretical temperature of the light emitting source. Midday sun in a cloudless sky is considered to be white light with a colour temperature of 5600º K. Artificial sources vary greatly in colour temperature with “warm white” incandescent lamps around 2500 K and “soft white” compact fluorescent lights are around 3000º K. The emergence of compact fluorescent and LED lamps has complicated white balance as the new technologies have different colour temperatures.
Even daylight is not the same colour. In the early morning and evening, the colour temperature lowers, giving a yellow colour to a scene, while grey clouds tend to produce a bluer or higher colour temperature. While it seems contradictory, the “warmer ” yellow colour temperatures are lower than the ‘colder’ blue colour temperatures and this is based our psychological reaction to the colours.
As well as the blue-yellow colour temperature regime, there is also a green-magenta tint component of White Balance in photography. The good news is that there are many aids for photographers in the camera and post processing to help get true colours in photographs.
AWB does this for cameras.
Digital cameras record the true colour of the light reflected from the scene. They have an adjusting mechanism called Automatic White Balance (AWB) that makes an adjustment to portray the colours as if the light is white.
This is a great advance over film cameras where you had to use a film formulated for the specific light conditions. The alternative is using filters mounted on the front of the lens to reduced the part of the colour range that would give a colour tint or cast in the photograph.
The camera value of White Balance is only a recommendation and is not applied until the image file is processed. Keeping the raw image file for later processing gives total control over the White Balance.
Sometimes help is needed
While cameras do an excellent job most of the time sometimes they get it wrong. And then there are scenes with mixed light sources. On these occasions, photographers need to help.
Many DSLR and advanced compact cameras have tools for setting the White Balance manually. When it is important to get the exact colour these tools are used to measure the White Balance. This is often the case in fashion or product photography where the colour is part of the story and near enough is not good enough.
To measure and set the camera’s White Balance involves taking a photograph of the scene with a large pure white object as the subject. This tells the camera that this is white and to use this as a reference for the White Balance.
For most photographers getting the exact colour right in the camera is not essential and is often easier to set in post processing using the raw image file. The White Balance is only applied when the raw file is processed. Processing in the camera and saving as a jpeg file reduces the options and control over the final White Balance of the photograph. The starting point for post processing colour temperature adjustment is the estimate by the camera when the photograph is taken, but this is more advisory than an essential part of the image.
White Balance adjustment
In raw file processing in Adobe Lightroom the White Balance can be adjusted to suit the photographer’s creative vision of the scene. There are a number of preset White Balance settings and a manual and automatic measuring tools for setting the White Balance. The danger here is that the colour displayed by the monitor may not be correct and so a colour cast is introduced when the photograph is printed or viewed on other devices. This highlights the need for a good correctly adjusted monitor for editing photographs.
If you are going to process the image in the camera to produce a jpeg then checking the White Balance is advisable because jpegs have less scope for adjusting white balance than unprocessed raw files.
Lightroom has a small pop-up menu in the white balance adjusting area offering a choice of standard preset White Balance settings. There are more for raw files than processed jpeg or tiff files. Remember, Lightroom uses non-destructive editing so any changes to White Balance does not alter the original image file. This allows experimentation with the different settings with the effect shown in the preview window.
Measure the White Balance
Lightroom has an eyedropper tool for selecting a section of the image as the colour reference for the White Balance. Even though this is all about White Balance the eyedropper is used on a grey section of the image and not white.
The R G B numbers at the bottom of the eye dropper loupe are a guide.The value of the number is not critical,what is important is finding a spot in the photograph where they are equal, or close to it. This is the perfect neutral grey to use as the colour reference. The scale slider at the bottom of the preview window controls the size of the selection area. Normally I use a graphics tablet for Lightroom, but for this function I prefer to use the mouse as it easier to position it precisely on a patch of grey.
What if there is not any grey in the photograph? The precise colour is not important, what matters is matching the RGB numbers at the bottom of the loupe. Adding grey by taking a test shot of a standard 18% grey card is a common technique for providing a colour reference.
In Lightroom this White Balance measured from the grey card with the eye dropper tool can be used for other photographs taken under the same lighting conditions. There are two methods, copying or synchronising.
This is selecting photographs from the grid view or the film strip and then clicking on one and then opening the Develop module. Make the White Balance adjustment and then click the sync button. When the Synchronise Settings popup appears check the White Balance box and uncheck all the others. This way only the White Balance adjustment is applied. Click on the synchronise button and the White Balance from the selected photograph that you adjusted is applied to all the highlighted photographs.
The Copy function is similar to Syncing except it is intended for one to one transfer of the White Balance setting between photographs.
When all else fails, or if one of the other methods is not close enough, there are adjustments for the basic colour temperature and the green/magenta tint. Drag the sliders or click on the number to the right of the slider and use the up and down arrow keys to for fine adjustment. Then adjust the image until the colour looks right on the monitor.
Lightroom Adjustments are Non-Destructive
The advantage of Lightroom and similar photo editing software applications is that the adjustment do not alter the original file. This means it is easy to experiment without losing any detail from the original. For White Balance, if you get lost and confused with different colour settings ii is easy to select the As Shot choice and start over.