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Thanks Jeff for your most helpful answer.
If you read on, you will see it's not really about Lightroom, it's about the fact that ACR 3.7, in either of its manifestations, has managed to screw up my white point settings in existing DNG files.
If I can understand how/why this happened, I might be able to solve the Lightroom problem (which was caused by ACR)on my own.
It is only fair to give an account of the process which led to the problem...
"Thanks Jeff for your most helpful answer. "
You're welcome...and LIGHTROOM (it is so very helpful to know what product you are talking about) actually uses the Camera Raw 4.0 pipeline-Camera Raw 3.7 was an interim update so Lightroom could read CR XMP metadata (and Camera Raw 3.7 could read LR settings).
With regards to Camera Raw 3.7, it would be useful to say HOW the .dng files were made, what camera you are using and how you've set up your Camera Raw defaults...
It should also be noted that Auto settings in Camera Raw changed with version 3.7.
As for questions about Lightroom, well, you know there's a forum for those now, huh?
BTW, as you might guess by now, there's even a DNG forum for questions specific to DNG.
Actually the product I am talking about is ACR 3.7, which is common to Photoshop CS2, Bridge and Lightroom. The Read Me! notes accompanying my installation of Lightroom clearly states that it uses ACR 3.7 (and that upgrading ACR 3.7 in Photoshop would be necessary to ensure consistency, which I have done). The reference to Lightroom was important, as unlike the Photoshop ACR, in my setup, Lightroom does not store the adjustment settings in sidecar XMP files, or, in the case of DNG files, within the files themselves. The problem manifested itself inside Lightroom first, and then, only after I upgraded Photoshop to ACR 3.7, did the problem occur within Bridge and Photoshop. This clearly demonstrates that the problem is ACR 3.7, and not some feature of Lightroom. [It might help therefore, if, in future, you forgot I mentioned Lightroom.]
To answer your next question, the DNG files were created in my camera (Pentax K10D firmware v1.0 & v1.1). ACR defaults are as out of the box except that sharpening is set to be applied in preview only. Data are "written to XMP" rather than a database.
The auto settings may have changed. However, the fault, when it occurs, is a major, and very obvious, shift of white point, not the sort of thing one would expect from a tweaking of the default/auto settings. Anyway it is only adjusted whitepoint settings, not auto ones, that have changed.
I cannot be sure that the problem is specific to DNG, though I suspect it probably is. My older PEF (Pentax RAW) files are unaffected.
I do not think it is acceptable that changes to software should cause changes to the rendering of an image, particularly where that rendering may have been created by careful adjustment. This is a fundamental issue of backward compatibility.
The default camera profile has changed for the K10D, which affects how previously applied white balance settings are rendered. You can pick to old camera profile to get the old rendering.
This raises an interesting question. How should one save a RAW file in order to preserve/protect the image rendering, and how should one protect it against the effects of future changes to the rendering software? Is it reasonable to expect to be able to do this?
As I have said, the alterations to some of my images were much more severe than one would expect from tweaking of the default/auto settings, and, if this was the reason, which is by no means obvious, then it shows a disregard for backward compatibility.
Thomas Knoll says that my camera defaults have changed. There is no way that the camera defaults could have been construed to produce the colours I have been seeing. A key point is that all the affected images had had their white points moved away from the As Shot values.
I answered our question. Rejecting the answer without even trying it is not going to solve your problem.
The problem is Pentax shipped the K10D with firmware that embedded a buggy embedded profile in the DNG. This embedded profile had the mapping between temperature/tint values and image colors way way off. (Pentax did the math wrong when they created the profile).
When I added official support the K10D in Camera Raw 3.7, I created a much better profile that has correct mapping between temperature/tint values. I made this the new default profile. I made the old (buggy) Pentax profile also available in the camera profile popup.
Unfortunately, when reading DNG files with existing settings created before 3.7, I did not correctly map settings to use the old profile, and it is using the new profile by mistake. This is the cause difference in colors you are complaining about.
To fix this, just pick in old profile from the Camera Profile popup. You can do this on muliple files at once using the filmstrip mode of Camera Raw (open all the files into Camera Raw, select all the files, and change the popup, and press done).
We try to avoid this kind of stuff happening. Sometimes we make a mistake (called bugs). This is one of those cases. In this case the bug was specific to Pentax K10D DNG files with settings adjusted before the camera was officially supported by 3.7.
The workaround (picking the old profile from the Camera Profile popup) is quite easy and fast. I suggest you just try it rather than just ignoring my answer.
Gottcha! Sorry, I was not meaning to ignore your answer, I just did not understand what it meant. (You only said that the default camera profile had changed, not how or why.) Now that you have explained it fully, I can see what has happened, and what I should do about it.
So, let's see if I have understood:
1. The problem only applies to DNG files created with Pentax K10D firmware v1.00, and...
2. which have had their white point adjusted using a version of ACR prior to v3.7.
3. The correct rendering of these older (v1.00) Pentax K10D DNG files requires maintained support for them within the RAW converter software.
The above accords with what I have found/believe, and what the camera default menu offers, but please correct anything that I have stated incorrectly.
I do appreciate your taking the trouble to explain this fully. I am also delighted that I can easily get my images back the way they were.