Relation between the Zone system and Histogram

Discussion in 'Digital Darkroom' started by guillaume_gaud_|1, Feb 18, 2010.

  1. Hi all,
    may someone tells me the relation between the zone system and the histogram (Photoshop for example) of a color JPEG 8bits image please ?
    I've come to these correspondences so far :
    [​IMG]
    Do you agree ? If not, can you tell me the exact relation please. I hope you understand my problematic.
    Many thanx in advance.
    Guillaume, Paris (France). Sorry for my poor english !
     
  2. Your English is great. I would disagree with your assertion, however. There is a correlation between a histogram and sensitometry, used in the Zone System. But the Zone System has more aspects, because it was meant to envision the tonal range of a scene, then the photographer must adjust exposure and development times to expand or contract the tonal range. A histogram really has nothing to do with the link between exposure and development, IMO.
     
  3. Guillaume,
    In addition to Michael’s points, you should be aware that not all JPEG RGB values are the same. It depends entirely on the gamma factor of the color space, and can vary dramatically between (e.g.) sRGB and ProPhoto. Your best bet would be to convert the RGB image to Lab space and then examine the L values. In Lab space, the L values are a very close match to the percentage of perceived brightness; L=50 a=0 b=0 would be a neutral middle gray.
    Cheers,
    b&
     
  4. Here is what I believe the relation to be. The 8-bit JPG values depend on the gamma encoding used. The two most common gamma encodings are gamma 2.2 and sRGB (linear up to 0.303% reflectance and then gamma 2.4 above that). I do not know which of the two is more common, but you can see that they are very similar except near black.
    Note that nearly all cameras will apply their own nonlinear tone curve to JPEGs in addition to the gamma curve, so to match these numbers you would probably need to use a RAW converter that allows a linear tone curve.
    Reflectance​
    8-bit gamma 2.2​
    8-bit sRGB​
    Slides Zone System​
    exposure compensation (stops)​
    100%​
    255​
    255​

    +2.47
    88.6%​
    241​
    242​

    +2.3
    72%​
    220​
    221​
    VII​
    +2
    36%​
    160​
    162​
    VI​
    +1
    18%​
    117​
    118​
    V​
    0
    9%​
    85​
    85​
    IV​
    -1
    4.5%​
    62​
    60​
    III​
    -2
    2.3%​
    45​
    41​
    II​
    -3
    1.1%​
    33​
    27​
    I​
    -4
    0.56%​
    24​
    17​

    -5
    0.28%​
    18​
    9​

    -6
    0.14%​
    13​
    5​

    -7
    0.07%​
    9​
    2​

    -8
    0%​
    0​
    0​
     
  5. Here is a link to a simple explanation to how the Zone System works.
    It includes digital Photography, which is very much like using it for shooting slides.
    They also have links to other sites you may find of value. As I have a disabled right hand, it makes no sense for me to repeat, what someone else explained better. :-D
    http://www.normankoren.com/zonesystem.html
     
  6. Why not define exposure in relation to actual data and where it should fall by the numbers in order to render a pleasing look. The numbers will be represented in a histogram which should only be used as a guide anyway. Don't rely on gray and white targets.
    I remember someone a while back noted for a good exposure to go by the highlights (not spectral) of colors similar to fair complected skin along the bridge of the nose, forehead and cheekbones checking to make sure the green channel RGB reading of this area be around 200 give or take. So this way you can apply this sort of "Expose To The Right" guide post number to prevent over exposure and blown highlights at the same time saturation clipping. Using a white or gray card wouldn't show saturation clipping.
    Since dynamic range is a constantly changing variable causing RGB data to squeeze and stretch up and down the tonal scale to get the scene to fit within 255 RGB levels whether normal, high key or low key with color temp inaccuracies affecting saturation clipping in whites, using the 200 green channel readout would be a better gauge to prevent clipping in both saturation and luminance viewed on a camera's histogram regardless of the color space and rendering engine. And the green channel most often represents the combined luminance averaging on a camera's histogram anyway.
    You could use a peach or beige colored target to gauge expose to the right so the histogram spike representing this color falls in this 200 RGB zone represented on the camera's histogram. Let all the other tones fall where ever they may fit within 255 levels.
    Just something different to consider I thought I'ld throw out there to see what you all thought. I mean I find f-stop, zones and reflectance measuring and readouts just too complicated and cumbersome to think about whenever I'm out shooting and trying to find the best exposure.
     
  7. @ Joe C : I agree with your tables Joe !
    As I shoot slides (Velvia 50 New and still Kodachrome 64), I have a flashmeter I use as often as I can. But in some tricky situations (eg. far illuminated subject when I'm in shadow, a rapid scene) I use the spot metering of my cameras F100 and F5.
    Most of my scanned images where I used spotmeter, I find about the same values as you Joe C : -1=85 0=118 +1=162
     
  8. I guess I'm confused at what everyone is trying to achieve. If it is to assign histogram values to zone values, that's fine, but how do you propose to consistently assign some value to zones between the visualization of the image and the expansion and contraction of the tonal scale in a digital file? Isn't the adjustment of the exposure and processing to get a print, with an envisioned tonal range, the point of using the Zone System?
     
  9. I tend to agree with Michael. Why tabulate what we can get without manipulation when manipulation is precisely the point? It makes much better sense to realize that nowadays we've got it all over Ansel Adams in terms of postprocessing capabilities, and to try to produce prints that don't show that Ansel has it all over us in terms of vision.
     
  10. Why tabulate what we can get without manipulation when manipulation is precisely the point?​
    I had assumed that the point was to know where each zone was in order to know where to compress or expand the tone curve when working with 8-bit files.
    Since the original poster is using actual slides, there will be some nonlinearity from the film, but the numbers will probably be close enough to be useful.
     
  11. Guillaume, your English is fine. But it would be helpful to understand the *context* of your question, i.e. why are you interested? Is it simply out of intellectual curiosity? Or, if you can establish the correlations between the two, how will you make use of them? My guess is that you want to get digital exposures using the histogram as you would with slides using the zone system.
    When shooting slides, I spot meter the highlight, and set the exposure manually to preserve its texture/details. It is a well established method of saving the highlights, at the expense of losing shadow details in contrasty images. When shooting digital, I can be more liberal with my metering methods, sometimes turning the control over to the camera's matrix mode, etc. If the histogram extends all the way to the right without clipping, I have achieved the same exposure as for a slide. They come up with a new term ETTR, as if it is something new. It isn't. But the histogram is new and very useful in confirming a correct exposure, something not found on a film camera. I don't worry about the correlations between it and the zone system, at least not to a great degree of detail.
     
  12. Thank you for your answers. I'm just coming back from India 2 days ago.
    I will precise my thoughts in few days.
    Guillaume
     
  13. From your figures Id figure that these numbers would also apply to slides.
    The first column, Zone II 7th column=Zone VIII the last Zone IX
    Zone I is black with detail on Film or Digital Zone IX White with detail.
    Zone 0 is black without detail. Zone 10 is glaring white, snow, etc.
    Note: the figures for Slides, which do not have the latitude of BW
    The zone system was created for BW basically. Zone I is Black, without detail.
    Zone X is White without details, and specular highlights.
    There is simplified information which may help more on
    http://www.normankoren.com/zonesystem.html
    along with charts, etc...
    [​IMG]
     

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