Why are Kelvin temperatures reversed between film and digital?

[saed]

hi

Why are Kelvin numbers reversed on digital cameras?
This is the opposite of Kelvin's theory in working with film.
For example, Kelvin 2000 has a warm color, but in a digital camera, the number 2000 indicates a cold color.

[kvbarkley]

Can you give us an example of what you mean? 

My T6S shows 3200 Kelvin for Tungsten - the same as film.

It shows Daylight at 5200 K, and daylight film is at 5500 K.

[saed]

The color of light is defined based on temperature in degrees Celsius, plus an absolute zero point in Kelvin. In simple terms, the lower the color temperature or Kelvin value, the light tends towards warm colors like red, orange, and yellow. As the Kelvin value increases, the light shifts towards blue and green. In the context of digital cameras, when the Kelvin value is high, the light appears warmer, and as the Kelvin value decreases (moves towards smaller numbers), the light becomes more towards the blue spectrum

[kvbarkley]

It is not the "Kelvin temperature", it is the black-body radiation which is measured in Kelvins. Note that at cooler temperatures, it starts radiating in infrared and as the temperature increases, the peak of the spectrum shifts higher from infrared to visible red to yellow to green to blue and on to ultra-violet. 

So at lower temperatures we can't see the infrared, so we see predominantly reds and warmer colors. At higher temperatures, the peak moves solidly into the visible wavelengths and we see a balance of colors with the blues dominating as you go even higher in temperature and the peak moves off into the UV.

[rs-eos]

Lower Kelvin values are warmer and higher Kelvin values are cooler.  That fact never changes.

Remember that the camera will always attempt to reach a proper white balance.   So if the current scene is actually at 5000º K, but you set to the camera to 4000º K, you're telling the camera that the scene is warmer than it really is.  So the camera will compensate by shifting towards blue.   Opposite if you set the camera to 6000º K.  You're then telling the camera the scene is cooler than it actually is, so the camera will compensate by shifting towards orange.

Same premise when using "reverse gelling".  The images I currently have for this experiment are that not great, so will just describe it and try to post some better images later.

The goal is to have a subject have a neutral white balance while the entire background goes warmer.  You light your subject (flash or continuous light) using a CTB gel (Color Temperature Blue).   Take a custom white balance photo (with the gray card completely illuminated by your blue light).   Load that photo as your customer white balance.  Then take one more photo.

Because your subject was too blue, the camera compensates by moving towards orange.  But will do so of course for the entire image.  The amount of orange added will neutralize your subject, but then for all background elements (or foreground if they exist), they will all now be warmer (orange).

[rs-eos]

I just re-did the reverse gel experiment.  Two light setup (600 EX RT with ST-E3-RT).  One on the background using a strip box and one on the subject using a 7" reflector with 10º grid.

All images taken with an EOS 5D IV with EF 135mm f/2.  ISO 100, 1/200 s, f/2.   Background flash always at 1/64 power.

Unless otherwise specified, all images were using a Kelvin value of 5000º.  And key flash was set to use 1/64 power.

Gel used was a double-strength CTB (Color Temperator Blue).  Light loss of the gell is 3.3 stops.  Mired shift of the gel is -260.

1. Initial black frame to ensure that no ambient light was affecting the scene.

Reverse Gel - Initial Black Frame

2. Key light only

Reverse Gel - Key Light Only

3. Background light only

Reverse Gel - Background Light Only

4. Both lights

Reverse Gel - Both Lights

5. Key light with double-strength CTB gel.  Power increased to 1/3 stop over 1/8 power to compensate for the 3.3 stops of light loss.

Reverse Gel - Adding the Gel

6. Capturing a gray card image for use with a custom white balance.  Power was reduced a bit of the key flash so as to capture a more saturated blue.

Reverse Gel - Gray Card

7. Final image using the custom white balance.

Reverse Gel - Final Image

[saed]

Thank you, my dear friend. This has been a challenge that many photographers and videographers who start their work with digital cameras may not grasp. I ask this question to all my students in every class, but they are not aware of it. In the end, I tell them it's like having a bag full of color temperature correction filters for the film. As you explained, these numbers inversely indicate the impact of the filter in changing the color temperature. I am very grateful for your time. Respectfully.