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Author Topic: Kelvin Temperature vs. RAW White Balance Slider  (Read 6911 times)
Edalongthepacific
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« on: July 17, 2011, 08:32:45 PM »
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The top RAW 6.2 White Balance slider goes from Blue 2000 to yellow 50000. I thought Kelvin lower was warmer (yellow) and higher cooler (Blue). What am I missing here?
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Graeme Nattress
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« Reply #1 on: July 17, 2011, 08:50:11 PM »
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The slider represents the colour temp of the light source the scene was shot under. That information is needed to make white appear white.

With light indeed, a 2800K light is a lot warmer than a 5600K light. But this is white balance - so say you shot a scene with 5600K light. 5600k light is pretty neutral to a silicon sensor, but 2800k light is not, it's very red (blue deficient) so if you have your 5600k scene and tell the raw development software via the white balance slider it was shot with 2800k light, the result will be very blue because the software is gaining up the blue to white balance a 2800k lit scene which will be very blue deficient.

Graeme
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Edalongthepacific
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« Reply #2 on: July 17, 2011, 09:28:53 PM »
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Got it, thank you.
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01af
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« Reply #3 on: July 19, 2011, 04:19:32 AM »
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The slider represents the colour temp of the light source the scene was shot under.
No, it doesn't ... or 'Edalongthepacific' would not have been confused.

Instead, it represents the correction required to pull an image shot under the given colour temperature back to neutral. That's why it's inverted.
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Graeme Nattress
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« Reply #4 on: July 19, 2011, 06:08:17 AM »
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Say I light a grey card with a light source I measure at 2800k. When I raw develop that shot I must set the kelvin slider to 2800k for such a grey card to appear neutral. Say I light a grey card with a light source I measure at 5600k. When I raw develop that shot I must set the kelvin slider to 5600k for such a grey card to appear neutral.

Hence the slider: "represents the colour temp of the light source the scene was shot under."
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elied
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« Reply #5 on: July 19, 2011, 10:26:26 AM »
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The scale number under the slider represents the temperature of the light source. The operation caused by the slider's postion, a very different thing, is the alterations to the red and blue channels appropriate to rendering an object that is neutral grey in reality as neutral grey in the image.
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bjanes
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« Reply #6 on: July 28, 2011, 09:13:32 AM »
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The slider represents the colour temp of the light source the scene was shot under. That information is needed to make white appear white.

With light indeed, a 2800K light is a lot warmer than a 5600K light. But this is white balance - so say you shot a scene with 5600K light. 5600k light is pretty neutral to a silicon sensor, but 2800k light is not, it's very red (blue deficient) so if you have your 5600k scene and tell the raw development software via the white balance slider it was shot with 2800k light, the result will be very blue because the software is gaining up the blue to white balance a 2800k lit scene which will be very blue deficient.

Graeme

This issue has come up many times on these forums, and I think Graeme's response is a good explanation. However, the temperature slider in ACR corrects for light emitted by by a black body radiator, which is closely approximated by the sun and incandescent bulbs. This adjustment is along the blue-yellow axis as indicated by the blue-yellow color of the ACR slider. However florescent bulbs and other light sources are not black body radiators and an extra adjustment along the green-magenta axis is needed (tint in ACR). See this Wikipedia article explaining the concept of correlated color temperature.

Regards,

Bill
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madmanchan
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« Reply #7 on: July 31, 2011, 07:52:28 PM »
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The scale number under the slider represents the temperature of the light source. The operation caused by the slider's postion, a very different thing, is the alterations to the red and blue channels appropriate to rendering an object that is neutral grey in reality as neutral grey in the image.

Exactly correct.
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