Let’s Talk Photography Ep.65 – White Balance

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Continuing his jargon-busting series of shows, Bart explains ‘White Balance’. You’ll find the full set of ‘Jargon’ shows at www.lets-talk.ie/jargon.

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Show Notes
  • The White Balance (WB) setting tells a digital camera about the colour of the ambient light — i.e. the colour of the light that’s shining on what ever is being photographed.
  • On cameras the white balance setting is usually defined by a collection of named presets like ‘daylight’, ‘overcast’, ‘incandescent’, and ‘florescent’.
  • In post-processing software the white balance is generally defined by two sliders, one for the colour temperature, and one for the tint.
  • Why do we need White Balance?
    • We see objects because light comes from a light source, bounces off the object, and then hits our eyes.
    • When our eyes sense colour they are sensing the frequency of the light that hits the retina after having bounced off the thing we are looking at.
    • Reminder from school — White light is light with equal amounts of all the visible frequencies, i.e. of all visible colours.
    • When white light bounces off an object that object will absorb some colours and reflect others. The combination of the reflected colours gives the colour of the object.
    • The real world is not illuminated by pure white light, so, the actual frequencies that arrive at our eyes are determined both by the colour of the light shining on the object, and the colour of the object.
    • The colours we experience are not the same as the colours our eyes senes. Our brains compensate for the colour of the source light.
      • If you take a piece of white paper out in the mid-day sun and measure the frequencies reflected you’ll record white light. The paper is white, and you measure white.
      • Take that same piece of paper outside at sunset, and measure again, what you’ll get is yellow. You’ll still see a white sheet of paper, but your eyes measured yellow.
      • You brain has adjusted your internal white balance!
    • Cameras measure frequencies just like your brain does. Specifically, digital cameras measure the ratio of intensities of green light, red light, and blue light and combine those three measurements to determine the frequency of the light for each pixel.
    • For a photo to look right the camera has to make the same kind of adjustment our brain makes, and compensate for the colour of the light that illuminated the scene.
    • Cameras need white balance to be able to reproduce realistic colour images.
    • The colour of each pixel in the final image is not the measured colour, but the result of combining the measured colour with the white balance setting.
  • Reminder — digital images store colours as ratios of red, green, and blue intensities, or RGB values.
  • Remember that when editing photos, the white balance is a combination of two things; a colour temperature and a colour cast more often referred to as a colour tint.
    • Colour temperature (Red to Blue):
      • Measured in degrees Kelvin
      • In physics, there is the concept of a perfect light emitting object known as a black body (‘black body radiation’). Black bodies emit light when they are heated, and the colour of the light shifts with the temperature. When you graph the intensity of the light from a black body against its frequency you’ll get a tall narrow mountain. The frequency at which the ‘mountain’ peaks shifts depending on the temperature — lower temperatures are more red, hotter ones more blue.
      • No object that exists in reality is a perfect black body, but most things that emit light because they are hot approximate one. When we heat metal if first glows a dull red, then a brighter red, then a very yellow red, then it starts to go blue. That’s the frequency of the peak shifting as the temperature goes up. Giant balls of glowing gas like the sun approximate a black body too!
      • You can describe a colour by the temperature a black body would be at if it were primarily emitting that colour of light, hence ‘colour temperature’.
      • When we say an image has a colour temperature of 5,600K, we mean the light that illuminated the scene came from something hot and glowing at a temperature of 5,600K, like say, the sun!
    • Colour Tint (Green to Magenta):
      • If you look at the colour wheel you’ll see that red is directly opposite blue. If you draw a line perpendicular to the blue to red colour temperature line through the centre of the colour wheel you get the green to magenta tint line.
  • Real-world White Balances
    • Remember, a white balance setting has two parts: colour temperature and tint. Saying an image has “a white balance of 5,600” is technically wrong, but generally interpreted as meaning a colour balance of 5,600K with no tint.
    • If you’re shooting in natural light there is usually no need to set a tint, you’ll just move the colour balance from warmer to cooler depending on the time of day.
    • Commonly used Artificial light sources have known colour temperatures and tints, hence, the white balance presets in cameras.
  • RAW -v- JPEG
    • A RAW image contains the original RGB values measured by the sensor as well as the WB setting to be used to transform those measured colours into the outputted pixel colours.
    • A JPEG image only contains the result of the White balance transformation, the original RGB values are discarded. (We say the white balance is ‘baked in’).
    • When you shoot RAW you can losslessly adjust the WB in post. When you shoot JPEG any adjustments will be extrapolations, i.e. educated guesses.
  • Grey Cards
    • A piece of material with a known colour, usually 18% neutral grey
    • When you place one in your scene you can tell your post-processing software that the grey card is a colour-neutral pixel, which allows the software to calculate the white balance of the original light (colour temperature + tint)
  • Artistic Considerations
    • You can choose to nudge the WB away from the scientifically correct value to alter the feel of your images
    • More red than the scientifically correct value is referred to as warm, and more blue as cold or cool.

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