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10-11-2019, 02:40 AM   #1
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White Balance - checking Kelvin value of a manually chosen setting

Looking at the WB panel of the K-1, I find it odd, that there seems to be no connection between the Manual WB setting 1-2-3 and the K setting 1-2-3. Is this true?
I had hoped, that a manual setting WB 1 would be the same as K1. Put in other words - I had hoped, that I could actually SEE the Kelvin value, I have set manually.
Is there a way to see this?

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10-11-2019, 05:12 AM - 1 Like   #2
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Manual White Balance Not Giving Temp - PentaxForums.com
10-11-2019, 06:49 AM - 1 Like   #3
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Read the discussion noted by K1N8, in particular my comments and those of photoptimist and stevebrot.

In particular, pay attention to the Planckian locus in color space. That shows what the R, G, B values should be for a particular black body temperature.

Essentially: for an arbitrary light source, when you set a manual white balance, the camera reads the R, G, B values it gets from the gray card exposure, which determines how much of each color there is from the light source, and determines a multiplier for each color so that they will come out equal (for white or gray) when processed later for an image. Gray = same amount of R, G, and B.

Those values can be anywhere in color space - if they are not on (or at least close) to the Planckian locus, then the concept of a color temperature for that WB setting does not make much sense!
10-11-2019, 08:54 AM - 1 Like   #4
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QuoteOriginally posted by Photodana Quote
White Balance - checking Kelvin value of a manually chosen setting
There are two other recent threads on similar topics, one just posted about 12 hours ago. The full answer is fairly complicated, but the short answer is that while the camera can use degrees Kelvin for white balance, calculation for AWB or manual WB does not involve degrees Kelvin.* Pentax is not alone in not providing an estimate from back calculation.

I might also add that a degrees Kelvin measurement would be made from incident light (the light falling on the subject) rather than light reflected by the subject. It is a fine point, but may be useful in understanding why a conversion is not offered.

Can I know the temperature set in a Manual White Balance profile (in camera)? - PentaxForums.com

Manual White Balance Not Giving Temp - PentaxForums.com


Steve

* White balance adjustment is required because the sensor is is tuned to a spectral sensitivity that corresponds to a standard light source at 5200° K and actual light might have a different spectral composition, usually not one that corresponds to black body emission spectra at a particular K°. Since the aim is to simply balance color, the camera calculates and applies correction factors to the processing of the RGB data from the sensor, no K° used and no K° needed.


Last edited by stevebrot; 10-11-2019 at 10:05 AM.
10-11-2019, 10:07 AM   #5
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10-11-2019, 10:08 AM   #6
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QuoteOriginally posted by Photodana Quote
I had hoped, that I could actually SEE the Kelvin value, I have set manually.
Is there a way to see this?
Is there a particular task you were hoping to accomplish if the K° were displayed?


Steve
10-11-2019, 06:13 PM   #7
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Using gels for balancing flash and ambient light

Hello Steve. The reason is a topic that has emerged in social media - how to choose which gel (color filter) to put on a flash, in order to color balance ambient light and flash light.
Most flashes are balanced to achieve light at appr. 5500. Knowing the Kelvin value / color temperature of the ambient light helps to use the right gel for the flash CTO or CTB:
Flash Photography Tutorial : How to use Color Correction Gels with Strobes - YouTube

10-11-2019, 08:10 PM   #8
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QuoteOriginally posted by Photodana Quote
Hello Steve. The reason is a topic that has emerged in social media - how to choose which gel (color filter) to put on a flash, in order to color balance ambient light and flash light.
Most flashes are balanced to achieve light at appr. 5500. Knowing the Kelvin value / color temperature of the ambient light helps to use the right gel for the flash CTO or CTB:
Flash Photography Tutorial : How to use Color Correction Gels with Strobes - YouTube
Oh! I guess that explains the sudden surge of interest in this. I am familiar with the video, but was unaware of any buzz associated with it. Most of what he says is very helpful, though he is a little fuzzy regarding color temperature and emittance spectra and also on what can be done with RAW capture. I don't know how many camera brands provide a Kelvin equivalent for manual/custom white balance. Sony (in the video) does. Nikon, as with Ricoh/Pentax, does not. If Canon does, it is not clear from their manuals, at least not for their higher-end product.

An option might be to iterate through available Kelvin degree WB settings until you get one that provides a nice balance to the ambient light. (Put a gray card in the frame for reference.) You can then use that value to assist in your gel selection and fine tune in Post. Where things get complex is when the ambient lighting does not fit conveniently into any of the discrete Kelvin degree temperatures. Stage and venue lighting are sort of famous for the ambient spectra being unlike any of the Kelvin degree emittance spectra. Manual/custom white balance can do a reasonable correction to neutral gray from a gray card, but a close gel match might not be possible.*

A more sophisticated approach is to make best effort for both ambient and gels and use a colorchecker (or similar) to make a color profile** for the session that might be applied later as part of RAW processing. This is done using a test shot that includes the colorchecker that is analyzed later to create a detailed color profile for that session and any others that might use the same lighting. Product shoots, particularly for clothing or fabrics, often use this approach.

Robert Hall mentions color meters and apparently is unaware that ambient light colorimetry is bread and butter for colorists in the motion picture industry. Prices for the meters start at over $700 and go up from there.

Sorry if this reads a little lame, but in practice, the gels are a much more rough approximation than manual/custom WB against a gray card, so I suspect that one's best effort is usually fine.

Steve

* How Sony does it and whether it always works for matching gels, I don't know.

** A good example might be session-specific DNG camera profiles (DCP) for Adobe and other products that support them.
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