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White Balance

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Gordon McLean
White Balance
on Oct 3, 2017 at 3:39:11 pm

Hi,

Long term lurker first time poster here!

I have a question that I have asked a couple of people and am yet to get a decent answer so thought I'd come here.

I understand colour temperature and normally set my white balance by setting the Kelvin but I often have a slight cast I have to fix in post. When I auto white balance it gives me the temp it has set to, so for a test I switched to manual and set to the exact same temperature. There is a definite difference in the image when i switch back and forth, manual has a slight green cast, even though no other setting has changed and the camera remains zoomed in on a white card.

So my question is does auto white balance adjust anything else other than the temperature and if not then why am I seeing a difference?

This is a general camera question but for the sake of this question I used a Sony NEX A 50H

Thanks


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john lenihan
Re: White Balance
on Oct 3, 2017 at 4:37:36 pm

Color balance. That could be a long long discussion.

It sounds like the number it tells you in Kelvin when it does the auto white balance is not accurate, or rather, not using the same calibration as when you set it manually. If I were you I would try this experiment, then find out which color temperature gives you the same picture tone. Then mentally use that offset number when you set it.

This error can easily happen if you are shooting with LED lighting, or possible two different colors of light, like daylight plus fluorescent.

Color in degrees Kelvin assumes black body radiation. In other words, a graph of energy vs frequency (which we interpret as color) would look like a bell shaped curve. Remember the curve in school.

This is true for sunlight and incandescent lighting. Your eye interprets the highest point of the curve, the average, as the color temperature of the entire light, even though other colors are present.

If you have LED lighting, it does not produce a bell shaped curve of light. It produces two spikes of light, usually one yellow and one blue. The automatic white balance tries to look at two spikes of color and guess what the average is. The average is actually between the two spikes. There is no perfect way for it to average two peaks and give you one answer that represents a bell shaped curve. So, the designers had to put in some kind of firmware to guestimate this.

John Lenihan

LeniCam Video Productions
http://www.lenicamvideoproductions.com


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Gordon McLean
Re: White Balance
on Oct 3, 2017 at 7:44:46 pm

Thanks for the reply!

It makes sense as there were indeed 2 light sources of different temperatures when I carried out my test. I will test again with a single non led light source and see what I find.

Thanks again for the informative reply!


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