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Influence of white balance in a digital sensor (2 replies and 7 comments)

rohith
3 years ago
rohith 3 years ago

Hi,

What happens inside a digital image sensor when we change the white balance.

        Any suggestions for some info on the process of how color determines the quality of an image in a digital sensor at the pixel level.

Thank you & All the best.

dmullenasc
3 years ago
dmullenasc 3 years ago

Technically nothing happens to the actual sensor when you do white-balancing.  The sensor itself is monochrome, it just responds to the amount of light hitting it.  The color filter array in front of the sensor, usually in a Bayer pattern, filters out certain wavelengths of color so that the value of red, green, or blue wavelengths per pixel can be recorded. This monochrome raw signal then has to be converted at some point into a color image.  White balance involves adjusting the signal level of each color channel individually (after conversion from raw to RGB) to create a certain color bias, or remove a certain bias.

Most sensors themselves have a "native" preference for near-daylight balance in that being less sensitive to blue wavelengths, they "prefer" a scenario where they receive more blue wavelengths, so when shooting in 3200K (tungsten) light, for example, in the conversion for raw, the RGB channels have to boost the blue signal more heavily than they would if you had shot in 5500K (daylight).  For the most part, the results are fine depending on the camera but if you looked closely, you often find that the blue channel is a bit noisier for 3200K scenes than in 5500K scenes if both were shot at the same ISO setting.  

In some ways, there is an analog film equivalent in that tungsten-balanced stocks have a faster blue layer to compensate for lower amounts of blue wavelengths in 3200K lighting, so if there had been such a thing as Kodak making both a 200T and 200D stock, or a 50T and 50D stock, you would mainly see that the blue layer would be finer-grained (i.e. slower) in the daylight stock.

rohith
3 years ago

Firstly, A Big Thanks.
>Why is it that most cinema cameras have their native balance set near daylight color temp, is it because of its ability to emit a continuos spectrum of light ( or) for any other reason.
>Correct me if i am wrong , the native color temp. of the sensor is where least gain is applied as whole , but there will amplification in appropriate R,G,B channels based on the corresponding color temperature values in order to show a grey card as grey in those lit conditions.

But the existence of various entities in the scene of the image behave differently with this process.

dmullenasc
3 years ago

The sensor isn't "set" to a native daylight balance, that's just what it naturally responds the best to because of a lack of sensitivity to blue wavelengths -- something to do with the nature of silicon or something, I don't know. I believe 3-sensor prism block broadcast video cameras like the Sony F900 were sort of artificially "set" to a native 3200K white balance by the way each sensor was filtered, but doing that in theory would be causing some extra loss of exposure to each sensor. Most of the gain boost would be in the blue channel when white balancing for 3200K, not so much in the red or green channels. But that's assuming you aren't also picking a higher ISO rating under 3200K to deal with low-light conditions.

This is why in the early Red cameras, which had problems with blue channel noise, some people were using blue filters when shooting under tungsten to reduce the noise in the blue channel or were just rating the camera at a lower ISO to reduce noise overall -- but either solution was problematic since tungsten-balanced situations tend to naturally be lower in light level than many daylight situations.

rohith
3 years ago

Thank You.
Are there any books etc., to dig deep into the prescribed subject. It would be of great help.

Thanks Anyway.

dmullenasc
3 years ago

David Stump, ASC wrote a good digital cinematography book.

Gregg
3 years ago
Gregg 3 years ago

Very generally speaking, what is the more common way to deal with colour balance when shooting digitally.  When you shoot film stock the white balance is either 3200K or 5500K depending on your film stock.  Does it make sense to limit the white balance setting on digital to either 3200K or 5500K and work in the same way that you would if you were shooting film?

Or if you are shooting outside with natural light for example, would you be changing the white balance on your camera as the colour temperature changes over the course of the day?  If you are dialling in the changing colour temperature, would you be colour balancing off a grey or white card, use a colour meter or change it based on known colour temperatures for the prevailing light conditions.

Obviously it depends on what look that you are after and whether or not you want to neutralise the shift in colour or use it artistically.  I am just interested to know what is the more common practise?

dmullenasc
3 years ago

It all depends on the nature of each camera in terms of noise issues and what ISO setting you are using. I believe with the Sony F55, you can only select 3200K, 4300K, or 5600K because of the processing it needs to do to convert raw to RGB for display.

For the most part, modern digital cameras shot at the ISO rating recommended by the manufacturer are clean-enough, noise-wise, to be shot at 3200K, so most common practice is to just work at the color temperature you want to work at and not worry about blue channel noise when using low color temperature values like 3200K and lower.

But if you were shooting against a blue screen, you may want to make sure your blue channel is not too noisy by either using a lower ISO setting in tungsten-lighting or by lighting for daylight-balance. On the other hand, many people shoot blue screens under 3200K lighting with an Alexa set to 800 ISO without much problem pulling keys.

How much noise is a problem is partly a matter of taste -- like salt in food, some people are more sensitive to it than others -- and partly a matter of how aggressive you have to manipulate the footage in post. For example, if you want super-saturated blues for some reason, it would be good to not be working with a recorded image that has a noisy blue channel because pushing the chroma in that channel will only make the noise worse.

Gregg
3 years ago

Thanks David. Very much appreciated.

Tomcam1
3 years ago

I Believe that the F-800 3200K color temperature was a "Throw Back" to previous Video Camera settings. Most TV studios were lit with 3200k fixtures. Because of the light levels engineers decided to balance the cameras  to 3200k so that there would be no need to place a color correcting filter in the optical path keeping as much of the original light levels hitting the sensors (ie.  way back,Tubes ) . Knowing that shooting out doors (5600K ) there would be plenty of light and the camera can be CC filtered with no compromising of the signal.

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