RAW vs White balance

Started Nov 20, 2006 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
K100D Dynamic range different at ISO 200/400...

I'm resurrecting this old thread, as I believe one of the OP's observations that there is something strange about the DS camera's exposures between ISO 200 and ISO 400 apply to the K100D and explain why Phil Askey's review of this camera showed more DR "headroom" at ISO 400 than at ISO 200. As follows:

GordonBGood wrote:

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'm not looking at the missing codes, which are clear indications
of bit shifting for ISO 800 and above, including ISO 800 as
Jonas B himself sees, too. I'm referring to the double width of
the ISO 200 histogram as compared to all of the other histograms
even though the exposure values were changed to compensate for the
higher differences in ISO by using half/double the shutter speed
with a fixed aperture. This was also the one of the observations
in the OP. It looks like ISO 400 may be accomplished as compared
to ISO 200 not by using a bit shift but merely be keeping the same
gain and changing the maximum white point value so that the raw
values will be scaled by a factor of two in the raw convertor
(which is basically the same effect as a bit shift).

The following images are "lifted" from Jonas B's post to this thread, with all credit due to him.

As the DS raw readings at ISO 200:

and for the DS raw readings at ISO 400 with half the exposure time but with otherwise identical settings:

Note that the width of the histogram which actually has meaningful data is something like a third again wider at ISO 200 than at ISO 400, where for a half the exposure time the histogram widths of meaningful data should be exactly the same width.

What we seem to be seeing here is that, when one sets these "D" cameras or the K100/110D to ISO 400, they are not actually increasing the gain of the signals coming from the sensor before conversion by the ADC by a factor of two, but rather just increasing the gain by about a factor of 1.5 with a tag to indicate that the range is now two thirds (or whatever the factor that was chosen was). I believe this explains why there is more headroom on ISO 400 and higher ISO sensitivities for these cameras than for at ISO 200.

It may also explain why those who use raw with any application (including ACR) that doesn't know how to read the tag indicating true gain or to make allowances for this characteristic of these cameras will find ISO 400 and above raw images to look under exposed and require positive exposure compensation.

This discrepency does not seem to apply to the new 10 MP K10D in that it appears that all pre-scaling is done in that camera using the new PRIME engine before the raw data is stored.

This is not specifically a problem as long as one is aware of it in that, first, one may enjoy the extra headroom and, second, boosting the exposure in post processing does not add any noise amplification that would not have been added in the same way by amplifying the signal before ADC conversion.

Regards, GordonBGood

Post (hide subjects) Posted by
w
w
w
ddd
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow