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Thread: Histograms Inconsistency: brightness, RGB, color, luminosity

  1. #1
    New Member
    Join Date
    May 2008

    Histograms Inconsistency: brightness, RGB, color, luminosity

    I recently became interested in flower photography and have not been particularly satisfied with my color reproduction. As I read up more, the tutorial at:
    Image Histograms, Part 2: Luminance & Color
    and others like it were supremely helpful for explanation of how one can clip one channel without noticing the clipping if looking at the "overall" histogram, which my camera manufacturer refers to as "brightness". I think "brightness" equates to "luminosity" in Sean's article, so this is one thing I'd want to see noted. More importantly, the article mentions that most cameras display a so called "RGB histogram" which would in fact show this clipping but I disagree. Most recent cameras seem to provide a choice of either:
    - brightness histogram (= luminosity, depending on your terminology)
    - RGB individual channels (referred to by manufacturers as RGB histogram, more confusion)
    I guess the bigger LCD ones may show both simultaneously, I don't know.

    Anyone out there still have cameras with a single "RGB" histogram?

    Histograms in general seem to be mis-understood by most. There are obviously various ones in your camera, Adobe Raw, Photoshop,...etc and they're not all the same. Moreover, blanket advice which is becoming more common about attempting to "push the histogram to the right as long as you don't clip" is troubling; if you are clipping one channel and care about that channel, your colors will be off. E.g. photographing yellow means you should pay utmost attention to green (usually OK) and red (very easy to blow out). I've been trying to photograph with the 3-channel view now, but that introduces its own worries. Ideally you want to make sure all 3 channels are not clipped on either side (or do you?) which is not easy (blue channel in particular always seems far to the left). I am interested in hearing other's experiences and what suggestions we'd give Sean to reword his tutorial to become more accessible to all and up to date with recent equipment. Do we need a 3rd part covering the photoshop various histogram choices?
    Last edited by osaman; 4th May 2008 at 02:54 AM.

  2. #2
    Join Date
    Apr 2008
    California, USA
    Real Name

    Re: Histograms Terminology: brightness vs. RGB vs. luminosity

    Thanks for your feedback on the tutorial on luminance and color histograms. As you rightly point out, there is quite a bit of confusion and inconsistency when it comes to terminology. When that article was originally written, most digital cameras I was aware of only showed one type of histogram, which was simply called "brightness" or "histogram".

    This histogram represented a sum of the number of pixels at each brightness level, for each color channel, irrespective of whether or not each of these RGB color values came from the same pixel. This is fast and easy to compute, and is in my mind the most useful way of showing a histogram if you can only display one kind (because it is better at showing color clipping). This type of histogram is also consistent with photoshop's default histogram (which they term as an "RGB histogram", but I will refer to this as a "photoshop-style RGB histogram").

    On the other hand, things have changed. Nowadays even mainstream cameras have the option to display information as one of at least two types of histograms--any of which may be labeled as a RGB, color, brightness or luminance histogram. Knowing which histogram your camera is displaying can be very important, because the difference between a photoshop-style RGB histogram and a luminance/luminosity histogram can be really pronounced:

    Histograms Inconsistency: brightness, RGB, color, luminosity

    On my Canon EOS 5D, for example, it can display either a brightness or an RGB histogram. Here's where the confusion starts: now my camera's label of "RGB histogram" no longer corresponds to the photoshop style RGB histogram, but instead represents an overlay or side-by-side view of histograms from each of the individual color channels. Further, the use of brightness is synonymous with luminosity/luminance in this case, and no longer shows a photoshop-style RGB histogram as any of the options.

    I'd like to present a rule of thumb, which I would like others to verify based on their experience with other camera models. This rule of thumb would state that if a camera can only show one type of histogram, then it is likely a photoshop-style RGB histogram; if the camera has options for two or more histograms, then one of these histograms will correspond to luminosity (and not photoshop-style RGB). What is everyone's experience with this? Any examples where it does not hold true?

    Note: The histograms tutorial (part 2) will be updated to clarify some of this confusion shortly...

  3. #3

    Join Date
    May 2008

    Re: Histograms Inconsistency: brightness, RGB, color, luminosity

    The CHDK hack for Canon compacts confirms that Canon compacts display a brightness histogram in playback and in record for the G-series cameras that display a live histogram.

  4. #4

    Re: Histograms Inconsistency: brightness, RGB, color, luminosity

    I'm using threshold layer (it uses luminosity histogram) in order to find the darkest pixel in the image and making tonal corrections according to it by using levels tool. Luminosity histogram shows very good distribution with no clipping at the shadow side. But, RGB histogram shows clipping at the same side. When distributing tonal range, which histogram should we use for better result?

    I have one more question: How can we mark the darkest and the lightest pixels by levels tool exactly?

    Thanks ...
    Last edited by Böğürtlen; 6th September 2008 at 07:51 PM.

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