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10-28-2007, 07:50 AM   #16
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Measuring impact of custom WB

QuoteOriginally posted by jeffkrol Quote
I tried to re-open this topic at dpreview and was met by silence...
Nobody care to re-open this topic???: Pentax SLR Talk Forum: Digital Photography Review
and


Re: RAW vs White balance: Pentax SLR Talk Forum: Digital Photography Review
I couldn't quite make out what was the consensus but it did seem to be "model dependent". But it does seem that some RAW has the custom WB applied directly to the data, it while normal "WB" is just a tag in the file. But I wasn't sure
I posted some to that origianl discussion. My approach to determining if there is an impact is to measure the tone curve of the RAW image and compare the tone curves under various conditions.

I originally did this with a VBA program in Excel. But it had a number of problems. I just recently re-wrote the tone curve program in Java. It produces three curves (one for each color) of the average raw values in a list of files. I use it by taking "white wall" images of a "pure" white piece of Epson Matte hea vyweight paper. I vary the exposure in steps of 1/2 stop from well below black to well above saturation.

From the curves I can tell a number of important things. Most important to me is how many stops from meter reading to saturation for various conditions.

I have only completed one sequence of white wall shots (for incadescent light at 6.8 LV).

Next step is to vary the WB settings (used AWB) and then do a custom WB to see if there are any changes to the curves.

Here is my first curve:



10-30-2007, 11:04 AM   #17
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QuoteOriginally posted by pentkon52 Quote
I create my CWB with filter on, and a shot of brightly lit green grass. I then get the mildly magenta'd image, as opposed to the AWB image that is mostly red in tint.
Similar effect as the first when using a white card and no filter.

Dave
Thanks Dave, that's what I was looking for.

!c
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