7 Replies Latest reply on Apr 9, 2011 12:25 AM by Vit Novak

    interpreting the histogram?

    southwestform Level 1

      How do you interpret the histogram? It shows me the color distribution, though what do you do with it? Do you use it while making adjustments or do you really eyeball the image? How is it used to be helpful?

       

      Thanks.

        • 1. Re: interpreting the histogram?
          Jeff Schewe Level 5

          The histogram is simply a graphical representation of the distribution of levels in your image. There is no "right or wrong" histogram. It's simply information. It can tell you whether or not you have clipping (either luminance or saturation) but clipping isn't always a totally bad thing. The histogram can tell you something about your exposure and the contrast range (how flat or contrasty). It can also give you a clue as to what adjustments to make but the appearance of the histogram is less important than the appearance of your image in the preview.

          • 2. Re: interpreting the histogram?
            Yammer Level 4

            Use the histogram to guide you but don't let it dictate what you do.

             

            The histogram represents the output of your conversion, rather than the source data. Your process settings will affect the histogram, as will the selection of colour space you are targetting.

             

            You can see from the histogram if your image is properly exposed and/or which colours are clipped, and maybe adjust your exposure/recovery/fill settings accordingly. You can also see if the balance of tones is as you would expect from your image.

             

            That's a very simplistic answer. Basically, it's like any other histogram except it shows you the effect of your settings, rather than the raw data.

            • 3. Re: interpreting the histogram?
              Yammer Level 4

              Sorry, Jeff. You posted while I was writing my response.

              • 4. Re: interpreting the histogram?
                Bill_Janes Level 2

                Jeff Schewe wrote:

                 

                The histogram is simply a graphical representation of the distribution of levels in your image. There is no "right or wrong" histogram. It's simply information. It can tell you whether or not you have clipping (either luminance or saturation) but clipping isn't always a totally bad thing. The histogram can tell you something about your exposure and the contrast range (how flat or contrasty). It can also give you a clue as to what adjustments to make but the appearance of the histogram is less important than the appearance of your image in the preview.

                As Jeff correctly points out, the histogram is simply a graphical representation of the distribution of levels in the image, but there are different ways to display the data. Sean McHugh has an excellent tutorial in his Cambridge in Color series:

                 

                http://www.cambridgeincolour.com/tutorials/histograms1.htm

                 

                The luminance histogram (the monochrome histogram on most digital cameras) analyzes each pixel in the image and weights the RGB components according to the sensitivity of the eye to each of the RGB components and the weighted sum is used to display the histogram. Since the human visual system is most sensitive to green, the histogram is largely determined by green.

                 

                The color histograms simply show the distribution of RGB in the image, but do not keep track of the color location of each pixel. The color histogram is valuable to show clipping in the individual color channels, but typically is shown after white balance has been performed. White balance involves multiplying each channel by a coefficient that is usually 1 for green and >1 for the red and blue channels. Accordingly, the color histogram may show clipping of reds and blue when these channels are intact. Nikon cameras allow one to set the white balance coefficients all to 1 (UniWB) to give a better inidcation of the actual status of the channels in the raw file. Clipping can still occur since the histograms are calculated from the preview image which is sRGB or Adobe RGB. ProphotoRGB would help avoid this saturation clipping, but is not offered by Nikon. I do not know how Canon and other cameras operate.

                 

                Photoshop does allow one to choose between luminance and color histograms.

                 

                Regards,

                Bill

                • 5. Re: interpreting the histogram?
                  Vit Novak Level 3

                  Bill_Janes wrote:

                   

                  ....  ProphotoRGB would help avoid this saturation clipping ....

                   

                   

                  Well, actually, very saturated blue colors can end up clipped also in PhotoProRGB (and consequently in sRGB or AdobeRGB). It happens with Adobe standard and older camera profiles, that are hybrid (matrix transformation + lookup table), because position of virtual blue primary of sensor color space is almost always outside PhotoPro, with negative y coordinate in CIE xy diagram.

                   

                  This was resolved in some new camera profiles, that have no matrix conversion

                   

                  Also, this was never a problem with Canon or Nikon cameras or software

                  • 6. Re: interpreting the histogram?
                    Bill_Janes Level 2

                    Vit Novak wrote:

                     

                    Bill_Janes wrote:

                     

                    ....  ProphotoRGB would help avoid this saturation clipping ....

                     

                     

                    Well, actually, very saturated blue colors can end up clipped also in PhotoProRGB (and consequently in sRGB or AdobeRGB). It happens with Adobe standard and older camera profiles, that are hybrid (matrix transformation + lookup table), because position of virtual blue primary of sensor color space is almost always outside PhotoPro, with negative y coordinate in CIE xy diagram.

                     

                    This was resolved in some new camera profiles, that have no matrix conversion

                     

                    Also, this was never a problem with Canon or Nikon cameras or software

                     

                    That is interesting and something I was not aware of. There was a long and contentious thread on LuLa that some blues in Adobe RGB might be clipped in ProphotoRGB, but the bone of contention was concerning chromatic adaption rather than the virtual blue of the sensor color space. Does the clipping to which you refer occur with real surface colors that occur in nature rather than with emmissive sources such as neon lights?

                     

                    http://www.luminous-landscape.com/forum/index.php?topic=49940.0

                    • 7. Re: interpreting the histogram?
                      Vit Novak Level 3

                      I'll try to illustrate this with a test image I used a while ago to make a camera profile for Canon 400D.

                      A part of this sRGB image with saturation 85% looks like this

                       

                      400Dtest1.jpg

                       

                      I displayed that image on sRGB CRT monitor, took several photos of it. Developping one of that raw files using ACR 4.4 profile in PhotoPro returns this

                       

                      400Dtest2.jpg

                       

                      If you check this image with some pixel picker or PS info tool, you can see that middle and dark blue has saturation 100%, so it's clipped even in PhotoPro, although original color was inside sRGB (and PhotoPro) gammut.

                      Matrix conversion in profiles is unfortunatelly only an aproximation and for some colors, resulting color is way off, although all patches on macbeth color checker using that profile can be ok (think about that when using X-rite or similar software).

                       

                      Of course, unlike ACR 4.4 which has only color matrices, other profiles (adobe standard, camera) have also lookup table. But it's after matrix conversion in the workflow, and what's clipped can't be unclipped ...

                       

                      Fortunately, this problem is obvious mostly with some colored fluorescent lighting, so this actually isn't a big issue, and as I said, it is addressed with some camera profiles (but not adobe standard), although ... rendition of sky is not particulary good with adobe standard profiles (underexposed sky is way oversaturated to my taste) and this is part of the reason ...