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UltravioletPhotography

Raw Therapee: white balance problem & work-around


Andrea B.

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I was just playing with the Raw Therapee app. It has lots of tools (lots of curves!), so I thought I should get to know it a bit. Seems like a good app overall for raw conversion. And it is free. Hard to beat that!!

 

Raw Therapee Links: in case you are interested.

Raw Therapee Features: http://rawtherapee.com/blog/features

Raw Therapee Wiki: http://50.87.144.65/...title=Main_Page

Raw Therapee Forum: https://discuss.pixl...are/rawtherapee

 

Raw Therapee UV White Balance "Problem" & Work-around

The Raw Therapee white balance dropper did not work quite as expected on my Labsphere white standard photographed in UV. It left a colour cast on the white area.

But Raw Therapee has a Channel Mixer. So I wondered if that could be used to white balance the photo? Short answer: Yes. And the channel mixer setting could be saved as a preset for batch use.

Now I am not claiming to have discovered anything new here. But there might be Newbies who could use this pointer when they want to apply a white balance to their UV photos and find their click-white tool does not quite do the job.

 

Original Photograph

As is well known, Nikons cannot set an in-camera UV white balance should you happen to want one. I have a reduced-red in-camera WB used for UV work which is shown here. I use it because it makes it easier to see what kind of result I'm getting in the UV photo while I'm shooting.

D600-broadband + UV-Nikkor 105/4.5 + BaaderU UV-Pass Filter

f/11 for 1/3" @ ISO-200

d600MohaveCalibration_20160221mohaveNatPresCA_44331.jpg

 

Raw Digger Raw Composite

Here are the actual colours (more-or-less) recorded in this UV photo. Disclaimer: Some changes may happen due to JPG conversion of the file and (ironically) stuffing it into an sRGB box. But it is close enough for all practical purposes. "-)

d600MohaveCalibration_20160221mohaveNatPresCA_44333rawComp.jpg

 

Photo Ninja Conversion + White Balance

PN's white balance dropper used on the white standard produces the expected neutral white tone.

No other edits.

d600MohaveCalibration_20160221mohaveNatPresCA_44331pnWbOnly.jpg

 

Raw Therapee Conversion + White Balance

RT's white balance dropper used on the white standard does not give a neutral white tone.

No other edits.

d600MohaveCalibration_20160221mohaveNatPresCA_44331rtWbOnly.jpg

 

Raw Therapee Conversion + Channel Mixer (R=65, G=95, B=120)

Now the standard is a neutral white tone. (I had to push the exposure +1 up after applying the Channel Mixer.) I will probably try to refine this Channel Mixer setting because I think I should not have adjusted the G channel? Usually WB can be achieved simply by adjusting Red and Blue around a fixed Green when using WB tools. I'm not sure whether that can be done with the CM tool. But the point is made that the CM tool can be used to find a white balance.

d600MohaveCalibration_20160221mohaveNatPresCA_44331rtChanMix.jpg

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