RAW vs White balance

Started Nov 20, 2006 | Discussions thread
CurtisR Regular Member • Posts: 263
Re: Unequivocal evidence - here too now

GordonBGood wrote:

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.

Um.. no. The WB is done pre-raw for preset WB as well. Go back and look again. I just did it with manual to show it very clearly (and demonstrate one potential practical use.) If you test with a brightly exposed target, the difference in preset WBs is clear as well - the russian guys show it well (and if you look closely at JonasB's shots it's clear too.)

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

I'm not sure what you're saying, but it seems clear to me that sensitivity up to, and including 800 are done without bit shifting (or you'd see black stripes in the histograms as you do at 1600 and 3200). I think you're misreading something here. And the "offset" that it seems you're talking about are just that there's no black in the target, and with the lack of a log function on the horizontal axis, its consistency gets exagerated when viewing these histograms. It does appear that whatever is done to achieve ISO 400 and above changes the response curve somewhat, but it's hard to say exactly how it is changed with only this data - something with two sets of peaks would be more informative.

I'm glad to see you using "counts" instead of "bits" as you did in your thread on the K10D sensor, but you keep posting this stuff that's a combination of true, incomprehensible, and materially incorrect. It's really a disservice to people trying to learn the facts.

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