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What is happening in the digital camera when white balancing?


Viggo Söderberg
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I'm writing an essay in school about white balancing in digital cameras and white balancing with color filters. We want to establish why digital cameras have a native white balance and why a camera with a native WB of 5600k has more noise in the image when balanced for 3200k.

I figured that we're talking about spectral sensitivity and that in general CMOS sensors are less sensitive to blue light. So when filming in daylight situations the sensor doesn't have to compensate the gain for the blue channel and thus not getting as much noise. Is that correct? Does anyone have a good source through which I can back these claims up, or if I'm wrong, what does control the native white balance?

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Yes, sensors are less sensitive to blue wavelengths so they work better in daylight.

In order to white balance a single-sensor camera, you have to debayer the raw signal into RGB - you have to do that to get any meaningful color image in the first place, white balanced or not. Then it's a matter of changing the signal levels of each color channel to balance.  Since the blue channel is underexposed in tungsten light, it has to be boosted and thus why it gets noisier than the other two channels.

With a 3-sensor camera, it can be designed so that the blue-filtered sensor is already amplified for tungsten balance, which is why with the old Sony broadcast camcorders, you actually had to go into the menus to get it to balance to 5600K, or use an orange filter.

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On 10/16/2020 at 7:14 PM, David Mullen ASC said:

Yes, sensors are less sensitive to blue wavelengths so they work better in daylight.

In order to white balance a single-sensor camera, you have to debayer the raw signal into RGB - you have to do that to get any meaningful color image in the first place, white balanced or not. Then it's a matter of changing the signal levels of each color channel to balance.  Since the blue channel is underexposed in tungsten light, it has to be boosted and thus why it gets noisier than the other two channels.

With a 3-sensor camera, it can be designed so that the blue-filtered sensor is already amplified for tungsten balance, which is why with the old Sony broadcast camcorders, you actually had to go into the menus to get it to balance to 5600K, or use an orange filter.

That's really interesting. I've noticed that the blue channel is usually the nosiest and just worked with a Sony camera this weekend and had to scroll trough the menu to change the white balance to 5500K. But never knew the reason why it was this way.

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