Magenta edge tinging at high ISO and a possible solution...

Started Aug 17, 2009 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: DCRAW/ACDSee pro 3 beta

photogerald wrote:

I noticed the following ISO 6400 shot was surprisingly free of the magenta cast:

http://forums.dpreview.com/forums/read.asp?forum=1036&message=33033827

When I commented about this, the poster said it was thanks to the DCRAW engine:

http://forums.dpreview.com/forums/read.asp?forum=1036&message=33038522

DCRAW is open-source, right? So I assume you guys have looked at the source for the latest version. I wonder what they might be doing (differently) to get rid of the magenta cast?

Gerald, yes, dcraw is open source and I just check Dave Coffin's latest for how it handles Pentax PEF files. No, it doesn't apply any sort of raw border correction as is the subject of this thread, and in fact it explicitly ignores the black masked border photosites when it decompresses the data. So I'm afraid the poster saying that dcraw corrects this was mistaken.

As I believe this border tingeing is due to temperature of the sensor, it is quite possible that the sensor may have been a little cooler for the shot on which you remarked. Unfortunately I cannot confirm this, as although the EXIF data is intact in the posted image, the MakerNotes are not included. There are also other ways to not see the magenta tingeing, mostly in the darker tones in tone curve manipulation in editing which will just "black them out". Finally, this was actually converted by the Acdsee raw converter which isn't definitely exactly the dcraw code and very likely has been re-written in extensive ways as it is much faster than dcraw implemented directly, and these changes may include its own dark border correction routines.

Conclusion: There may or may not (depending on sensor temperature?) have been border tingeing in the original raw source file that may have been removed by Acdsee's raw converter, either through manipulation of the tone curves or through a black border correction that is applied by the Acdsee version of raw conversion (which may or may not include much in common with dcraw).

Regards, GordonBGood

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