Understanding the Histogram

This is a discussion on Understanding the Histogram within the Post-Processing forums, part of the Photography Tips category; Originally Posted by twinkle_turnip DS- Why is it that there are more hilight tones than shadow tones? As far as I knew the palette is ...


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  1. #11

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    Quote Originally Posted by twinkle_turnip View Post
    DS- Why is it that there are more hilight tones than shadow tones? As far as I knew the palette is distributed evenly over a 2^(bitdepth) gradient. The camera doesn't know anything about what a "hilight" is, only that the sampled analog value of any given position on the CCD is a specific current that correlates to a specific binary value. Is the placement of levels nonlinear???

    From what I always understood is that because the CCD cannot measure the absence of light, but rather the amount of light present, brighter pixels of higher analog current produce more useable signal relative to inherent noise present in the system and as a result the digital sampling is more accurate than a dark pixel which of lower analog current. This, I thought, is why slight over exposure produces better results.
    I am interested in reading the answer to your question too T_T but whatever the answer may be I am thinking what we need to know for practical purposes is much simpler -
    The Histogram - QUOTE
    "It is a simple graph that displays where all of the brightness levels contained in the scene are found, from the darkest to the brightest. These values are arrayed across the bottom of the graph from left (darkest) to right (brightest). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level."
    quoted from ~

    Understanding Histograms

    Good Read

    Interesting thread IMO.
    Thanks to all.


    KimR

    Comments and Critique are always encouraged, considered and appreciated. Thank in advance too.

  2. #12
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    Quote Originally Posted by twinkle_turnip View Post
    DS- Why is it that there are more hilight tones than shadow tones? As far as I knew the palette is distributed evenly over a 2^(bitdepth) gradient. The camera doesn't know anything about what a "hilight" is, only that the sampled analog value of any given position on the CCD is a specific current that correlates to a specific binary value. Is the placement of levels nonlinear???

    From what I always understood is that because the CCD cannot measure the absence of light, but rather the amount of light present, brighter pixels of higher analog current produce more useable signal relative to inherent noise present in the system and as a result the digital sampling is more accurate than a dark pixel which of lower analog current. This, I thought, is why slight over exposure produces better results.
    Don't know, don't build them. It has to do with the file structure. There are 5 brightness levels, name them whatever you want, but the fact remains that the files are structured that way so obviously they are detectable.

    ...A 12 bit RAW file is capable of recording a theoretical maximum of 4,096 levels of brightness, the amount of data allocated to the different levels of brightness within the scene varies, with half [2,048] allocated to the brightest areas, half of the remaining 2,048 to the next area [1,024] ...(and so on). This means considerable more data is used to record detail within the brighter areas of the image...

    Practical HDR
    David Nightingale
    Capture all the photons you want; the universe will make more.

  3. #13
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    Already covered
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  4. #14
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    It turns out you're right, DS, but I think you are misinterpreting what is being said. Brightness is measured relative to other tonal areas in terms of EV or Stops whereas the camera measures light linearly.

    From Kim's suggested reading:

    QUOTE
    Let's assume for the purposes of illustration that a digital SLR has a dynamic range of 5 stops (it's usually closer to 6 stops, but let's not quibble). When working in RAW mode, which you should be, most cameras record a 12 bit image. (Yes, we say it's in 16 bit mode, but the reality is that it's only recording 12 bits in a 16 bit space. Better than 8, but not as good as a real 16 bits would be).

    A 12 bit image is capable of recording 4,096 (2^12) discrete tonal values. One would think that therefore each F/Stop of the 5 stop range would be able to record some 850 (4096 / 5) of these steps. But, alas, this is not the case. The way that it really works is that the first (brightest) stop's worth of data contains 2048 of these steps — fully half of those available.

    Why? Because CCD and CMOS chips are linear devices. And, of course, each F/Stop records half of the light of the previous one, and therefore half the remaining data space available. This little table tells the tale.
    http://www.luminous-landscape.com/tutorial...ose-right.shtml

    This has less to do though with file structures or engineering choices as it has to do with "brightness" and the inconvenient truth that our photographs are not "made" of light, but rather numbers that represent light. I can kind of visualize it, but I am having a really hard time finding the words to describe it as I had just learned about this concept now.
    bear with me. i don't have an escape button...

  5. #15

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    Quote Originally Posted by DSRay View Post
    Quote Originally Posted by twinkle_turnip View Post
    DS- Why is it that there are more hilight tones than shadow tones? As far as I knew the palette is distributed evenly over a 2^(bitdepth) gradient. The camera doesn't know anything about what a "hilight" is, only that the sampled analog value of any given position on the CCD is a specific current that correlates to a specific binary value. Is the placement of levels nonlinear???

    From what I always understood is that because the CCD cannot measure the absence of light, but rather the amount of light present, brighter pixels of higher analog current produce more useable signal relative to inherent noise present in the system and as a result the digital sampling is more accurate than a dark pixel which of lower analog current. This, I thought, is why slight over exposure produces better results.
    Don't know, don't build them. It has to do with the file structure. There are 5 brightness levels, name them whatever you want, but the fact remains that the files are structured that way so obviously they are detectable.

    ...A 12 bit RAW file is capable of recording a theoretical maximum of 4,096 levels of brightness, the amount of data allocated to the different levels of brightness within the scene varies, with half [2,048] allocated to the brightest areas, half of the remaining 2,048 to the next area [1,024] ...(and so on). This means considerable more data is used to record detail within the brighter areas of the image...

    Practical HDR
    David Nightingale
    It may be easier and practical to think of the histogram we use on camera or in pp to be a graph of 256 stops with each of those stops representing many discrete brightness levels (about 50 on a histogram of 8 bit). The 5 brightness levels you speak of can be thought of as from the left starting with darker, dark, medium gray 18% (center), light, and lighter which are all divided equally on the 256 points of the histogram. If 8 bit, 12 bit, 14 bit or ... more each of the points 0 to 255 represents a point with differing amounts of discrete brightness levels as the size of the file (bits) change however the histogram still works the same.
    KimR

    Comments and Critique are always encouraged, considered and appreciated. Thank in advance too.

  6. #16
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    See, and this is how I used to think about it, but that stupid link you gave me threw everything off!

    The problem is that those steps of luminance don't correlate directly to the amount of exposure at the sensor since brightness is exponential and luminance is linear.
    bear with me. i don't have an escape button...

  7. #17

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    Quote Originally Posted by twinkle_turnip View Post
    See, and this is how I used to think about it, but that stupid link you gave me threw everything off!

    The problem is that those steps of luminance don't correlate directly to the amount of exposure at the sensor since brightness is exponential and luminance is linear.
    T_T

    Not laughing at you, just laughing...

    I have to disagree and say there is really no problem unless of course you create one. The histogram is a very valuable tool, the histogram works very well but... don't think Mathematics vs Linear, merely simplify complications and think
    ... to dark, to light and everything in between and you will do fine.
    This was kind of a fun thread and good mental exercise for me. Thanks
    KimR

    Comments and Critique are always encouraged, considered and appreciated. Thank in advance too.

  8. #18
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    Quote Originally Posted by KimR View Post
    Quote Originally Posted by rockhead View Post
    I have to agrees with the above.

    Clipping indicates loss of information. You can not recover this information by any editing process. So if your intention is to retain all that information in a scene you should try to capitalize on the histogram being exposed mainly to middle gray with the primary information radiating from the center of the histogram in light and dark directions. Most the time you'll find you don't need all the info because frankly most smooth middle gray exposure will be flat.
    But if you shoot to middle gray you should be able to edit which way want with out too much loss. And the histogram is a great tool to use making curves and levels adjustments.

    For a small experiment get a gray card shoot a picture of it set as your white balance.

    Set your aperture to say f8 in aperture priority mode 0 exposure compensation.
    Now turn down you compensation -2 ev take a picture it should apear black or really dark gray
    Now turn your exposure compensation up to -2 ev it shot appear white or really light gray.
    You must use a tripod and not change the lighting.

    Take those 3 shots to your editing program look at the histogram for the 0 exposure all the info should pretty straight up the middle. Look at the histogram for the -2 most the info should be bunched up on the left. Look at the histogram for +2 most the info should be bunched up on the right.

    Now edit the dark shot to middle appear middle gray and edit the light shot to appear middle gray. Use a eyedropper tool to sample the color it should read 128(middle value from 0 black 255 white)Look at the histograms for both edits do you see the same shaped histogram as the middle gray?????

    Likely you dark edit will increase noise and your light will have less noise but gaps in the histogram like the dark.
    If I had my old hat of ten years ago on (my hat of beginning digital photography) I would not question what you have written rockhead. I lost that old hat though and would never agree with "try to capitalize on the histogram being exposed mainly to middle gray".

    Expose to the right is my recommendation for capitalizing on exposure and you can read why here ~
    Expose (to the) Right

    Thanks for the interesting link.

    In practice I push white as far as i can.
    The idea was to not clip important information. The exercise was an simple example of what happens as you edit from dark to light. Instead of exposing correctly.
    My rambling just simplified for someone starting out which the O.P.'s intention of the thread.
    In my second post I confirmed my practices and beliefs which are in line with your own.

    Keep
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    stupid
    Is the name of my game.

    Just so you know your right I STAND CORRECTED BY KIMR. It seems to be a lot more important to you than me This line is a joke also hard to convey via internet connection.
    Get OUT and SHOOT Y'ALL...........Sitting at the 'puter don't make better photos!!!!!!

    Some of my crappy photography on FLICKR

  9. #19
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    Kim- I don't know where your background is, but mine is in traditional ZONE SYSTEM black and white photography. So correlating zone placement with histogram placement is important.
    bear with me. i don't have an escape button...

  10. #20

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    I have a friend down in Florida who shoots purposefully to left on the histogram and does some amazing imaging and processing. He is not doing anything wrong by slightly underexposing and he has a unique style, the result he wants and gets are pretty interesting and cool and noise does not seem to be and issue for him either.

    When I first started shooting digital there were a lot of us just starting to shoot digital as digital cameras had not been out long. At that time and for quite some time there was many of us that tried to actually shoot exposing a bit to left of center on histogram, then as we shot more we began to realize that we had been shooting on the wrong side of the line. Folks passed it on, we talked about it one to another and in groups and we all got to learn from those talks and experimenting with different ideas. It was not about being right, it was not being wrong but it was about sharing, learning and teaching others too. For me today the sharing and learning is still the what it is about.
    KimR

    Comments and Critique are always encouraged, considered and appreciated. Thank in advance too.


 
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