RGB Histogram question

Discussion in 'Beyond the Basics' started by m1a1fan, Apr 22, 2008.

  1. m1a1fan

    m1a1fan TPF Noob!

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    I have a question concerning RGB Histograms.

    While out shooting in Arches National Park and Capitol Reef, I was paying a lot of attention to my RGB (as well as my general luminosity) Histograms and noticed that while most of the exposures I made fell within the 0-255 range of the above Histograms I did notice instances where one or two of the RGB channels “blew out” or exceeded the range of the camera’s sensor. Adjusting the exposure often corrected one of the channels but not usually both.

    My guess is that one would have to properly expose the channels that are correct and then make another exposure for the channel that is “blown” properly exposing it and then blending the exposures in Photoshop.

    Am I thinking down the right track here? I’m beginning to gain a better understanding of exposure and getting this little piece of info correct will greatly help.

    Thanks for the help guys and gals-
    Ryan
     
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Not quite. The RGB histogram is very dependant on the type of RGB encoding used in the file. For instance the value of red 255,0,0 in sRGB corresponds to only 218,0,0 in AdobeRGB. It depends firstly on how the histogram displays colours. Often if you open up the RAW file which has one channel such as red clipped, you will see the red blow out to yellow on certain tones. This starts dramatically changing if you export the RAW file as an AdobeRGB or as sRGB etc. And when you start converting between them things change again.

    The messy point I'm making here is it depends on the colour management you use. But often if you have one channel clipped it can be recovered in any RAW processor. Lightroom's and Bridge's "Recovery" slider makes very short work of a single clipped channel.

    If you try to make two different exposures for 2 different channels it will horribly screw up the colour when you try and combine them afterwards.

    If you shoot in JPEG however it becomes important to watch the clipping of individual channels as when one channel clips the others catch up and the hue starts to change.
     
  3. m1a1fan

    m1a1fan TPF Noob!

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    Garbz-

    Ah, so it is a bit more involved than I had first thought. Thanks for taking the time to reply. Is there any good reading on this subject? Or is this something that is just learned with time?

    Thanks-
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    There is but I can't think of any complete all in one resource. There are a few good articles on colour space and histograms on www.luminouslandscape.com but I can't think of any one article that tied the two together. I figured most of that out by playing with the colour settings in lightroom and photoshop. My monitor can display the extended gamut so I can see the difference when I play with such things.

    Ultimately though the final destination is usually the limiting sRGB space which is the standard. As lightroom or photoshop or any other colour managed application will display the picture in accordance with your monitor profile (sRGB) you should see clipping in colours which can be reduced by moving the saturation slider or the brightness slider. If this were true clipping (RGB all at max) then the highlights would not be recoverable, but often they are.

    I also believe that Adobe's Recovery feature in Bridge and Lightroom uses this fact too as it is able to rebuild detail in the highlights providing one or more of the channels are not clipped all the way.
     
  5. kellylindseyphotography

    kellylindseyphotography TPF Noob!

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  6. Mav

    Mav TPF Noob!

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    Ryan - what were the settings on your D80? If you were in any of the Vivid modes, I'm pretty sure that it manually jacks the contrast up higher, which might make it more difficult to get a proper output with all of the channels reeled in if you have a particularly colorful and/or contrasty scene. If that was the case, you might have been better off leaving it in the Normal mode (auto contrast and saturation) or a custom setting with saturation up, but contrast (tone compensation) still left on autopilot. That way if you have a really contrasty scene, it'll automatically lower the contrast so that you can still get a reasonable exposure. It also depends on the scene, but a lens filter of some sort may have helped, like a circular polarizer, or a graduated neutral density filter (for a bright sky but much darker at the horizon and below).

    Posting an example photo or two with EXIF data intact would be of great help.
     

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