RAW vs White balance

Started Nov 20, 2006 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: Unequivocal evidence - here too now

Jonas B wrote:

Now I have to think for a while about this. In what situations can
one benefit from this? NB: The metering doesn't change. With the
pink towel exposed with a manual white balance the red channel is
clearly held back.

This does confirm what was stated in the Russian article that WB does change the relative gains of the colour channels, but as per your earlier tests and with these, only manual white balance affects the gains. There is an advantage in this in that using manual WB will avoid clipping of the red channel in cases with extremely warm light, as in tungsten or candlelight. One might assume it might also avoid blue clipping in cases with extremely cool light as in extreme shade, etc., although this would be less often a problem. Not bad, just interesting.

The second part of the observations by the OP was that there is a discrepency in gain for all channels between ISO 200 and ISO 400. As per your earlier post with the multitude of charts, it looks like this is true in every setting for WB (I would assume for Custom WB as well). With my knowledge of the Pentax raw format for the K100D, I assume that there is the same offset on all channels in raw of about 128 counts, so what we seem to be seeing here is that the ISO 400 charts are just reduced by a factor of 2 which is just another form of bit clipping - not using the upper half of the range. I'm sure that if one looks in the raw file, they will find that the stated maximum range reflects that the gain has been reduced. In other range, there is just one ISO sensitivity setting that is the real gain of the pre-amplifier and ADC: ISO 200. It looks like all of the others are just software manipulated, first just amplified by the maximum range setting for ISO 400, then just bit shifted for each subsequent increase in ISO by a factor of two.

I suppose this is done for every "D" model, and very likely the K100/110D, but it would be interesting to confirm. It doesn't appear to be done that way for the K10D from what I see in the DNG samples, but then they have that huge range of the 22 bit ADC to play with so it wouldn't be necessary to do it in the same way.

Regards, GordonBGood

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