11/08/2015
NOTE : White Balance.
The easiest way to describe white balance is by way of colour temperature which in itself is a way of measuring the quality and intensity of a light source. This is based on the ratio of blue and red light hitting the sensor, with the green light being ignored.
You may sometimes see a red or yellow cast in your images when shooting indoors with natural light? This is caused by using the incorrect white balance. To correct these casts in Photoshop, go here.
The unit for measuring this ratio between red and blue is known as degree Kelvin or K. Therefore, a light scenario with a higher colour temperature such as bright, sunny blue skies has more “blue” lights and a higher Kelvin Value. Whereas a light with a much lower colour temperature such as a candle flame, has a lower Kelvin value with more “red” lights.
The following values are approximate but should give you an idea of how this is seen in most photographic situations.
Light Source Colour Temp in K
Clear Blue Sky 9,000 to 15,000
Overcast Sky 6,000 to 8,000
Noon Sun and Clear Sky 6,500
Sunlight Average 5,400 to 6,000
Electronic Flash 5,400 to 6,000
Household Lighting 2,500 to 3,000
200-watt Bulb 2,980
100-watt Bulb 2,900
75-watt Bulb 2,820
60-watt Bulb 2,800
40-watt Bulb 2,650
Candle Flame 1,200 to 1,500
The human eye is perfect at adjusting to these fluctuations and will see a piece of white paper, for example, as white whether you look at it outside in bright sunshine or inside by candlelight.
Your camera, on the other hand, will have more difficulty.
When using emulsion films, you have a choice of using daylight or tungsten sensitivity, but with digital, most adjustments need to be made “in-camera”.
Most digital cameras have built-in sensors or “Auto White Balance” to measure the current colour temperature and then use an algorithm to process the image correctly.
The final result may very well be close to what we see with our eyes, but the algorithms being used may not be accurate enough to make every situation or image correct.
If you or the camera set the temperature or WB incorrectly, you will notice some colour shift or “cast” in your pictures.
For example, if you are shooting indoors under normal household lights but set your white balance to that of outdoors, your camera will expect excessive blue light, less red and adjust accordingly. However, the light from most bulbs has a low colour temperature or “K” value thereby having more red light than blue.
The resulting image will have a reddish or yellow appearance which can be corrected by either going to manual (white balance) on your camera, and reducing the WB setting until correct or playing around with the colours in an editing program during post processing (more on that later).
Similarly, if you set the cameras white balance temperature low to around 2,500 and take a shot outdoors, the camera will expect more “red” light and adjust its algorithms accordingly. Of course, the actual scene has more blue light and less red and the result will be an image with a cool, blue look to it.
The beauty of digital photography means that you can…
1.Take a test shot
2.Check the white balance
3.Adjust accordingly
4.Take another shot
5.Delete all tests
6.Get the picture right
For most situations, your cameras AWB (Auto-White-Balance) mode is good enough. Any minor fluctuations can be dealt with during post-processing although there may be times when you need to get out of your “comfort zone” and go manual.
