JackProton
sage
Reged: 03/01/06
Posts: 314
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This past winter, I tried using a didymium glass/red enhancing/intensifier filter to cut some of the local light pollution. Didymium glass is made from a mixture of rare-earth elements and excels at blocking the spectral lines of sodium without affecting the rest of the visible spectrum. Photographers sometimes use the filters to selectively enhance reds in a way that isn't easily duplicated. At the time, I only had a 35mm camera. Compensating for the 1/2 to 2/3 f-stops the filter introduced, I couldn't find much, if any, difference with or without the filter.
Now that I have a Rebel XT, I thought I'd try the didymium glass filter again. The results this time were quite surprising -- the usual reddish-brown mist from light pollution was COMPLETELY missing. In light of the previous test, the difference seemed too great to be explained by the use of the filter alone. Running some daylight tests, I found that the Auto White-Balance in the Canon Rebel XT was compensating for the reddish tint introduced by the filter and graying out the background light pollution in the process.
So, in terms of testing the usefulness of the filter, it seems like I should use a fixed white-balance setting such as SUNLIGHT. In terms of actually using the filter, if it should prove useful, I'm not certain how I should set the white-balance. Any suggestions?
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cweinhei
sage
Reged: 01/14/06
Posts: 414
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I wonder if photos taken in RAW mode are subject to white balance? Do you know?
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Astrolupa
member
Reged: 07/14/05
Posts: 82
Loc: Algarve, Portugal
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Photos taken in RAW mode are not subject to white balance nor other kind of processing, that's why their RAW's 
Seriously... You'll notice that after you convert from RAW to a more convencional format, IF the conversion itself does not do any kind of extra processing, your photos will look very greenish. That's ok, because it represents the reality, as the sensor have twice the green pixel count than red ou blue. After processing you can apply a white balance.
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Fabio Papa
Carpal Tunnel
Reged: 03/01/05
Posts: 1734
Loc: Piacenza, Italy
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More on what Astrolupa said, you must pay real attention at the software which converts RAW to other formats, as they tend to apply white balance while doing the conversion. I heard Canon software will do that, as I think some plugins for Photoshop. Needless to say that such procedures takes away many of the benefit of taking raws in the first place.
I use IRIS for the conversion. Even if it's complex, I am 100% sure it doesn't touch the original data.
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Fabio Papa
Celestron C8 F/10
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2020BC
scholastic sledgehammer
Reged: 08/03/05
Posts: 984
Loc: Australia
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Quote:
...it represents the reality, as the sensor have twice the green pixel count than red ou blue. After processing you can apply a white balance.
What does this mean, when you say, "..the sensor has twice the green pixel count..." ? 
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krneki
super member
Reged: 12/24/05
Posts: 156
Loc: sLOVEnia (45.5° N)
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He's referring to the bayer mask.
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Astrolupa
member
Reged: 07/14/05
Posts: 82
Loc: Algarve, Portugal
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Thanks krneki, that's really it. I was refering to the Bayer mask.
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Best Regards,
Nelson Viegas
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LXD55 10"
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JackProton
sage
Reged: 03/01/06
Posts: 314
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Quote:
More on what Astrolupa said, you must pay real attention at the software which converts RAW to other formats, as they tend to apply white balance while doing the conversion. I heard Canon software will do that, as I think some plugins for Photoshop. Needless to say that such procedures takes away many of the benefit of taking raws in the first place.
I use IRIS for the conversion. Even if it's complex, I am 100% sure it doesn't touch the original data.
Good point. Some of the Canon software and the Photoshop plug-in do allow you to select a different white balance setting from the camera settings stored with the raw data though. I'm not entirely sure, however, that the raw data can be processed without some sort of white balance.
I wonder which white balance setting is appropriate when using this filter though. Using the automatic white balance settings stored with the raw data will give me something that appears, more or less, "natural" looking, however, I can't help wondering if this procedure merely lowers the red levels and is actually counterproductive for detecting patches of faint nebulosity.
On top of that, I think I've totally confused myself how to make a proper comparison between images made WITH and WITHOUT the filter. Too many options!
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