Histogram Anomaly

Discussion in 'Photography' started by Wilba, Mar 8, 2007.

  1. Wilba

    Wilba Guest

    I've been trying to learn more about how to make effective use of the
    histogram while shooting, and as part of that I took a bracketted series of
    the view out my back door (nothing special, just the handiest outdoor scene
    at the time).

    The images are here - http://people.aapt.net.au/~wilba/histograms/ - with
    pre-resize levels from Photoshop Elements. Olympus C-770, f6.3, 1/250s, ISO
    100, aperture priority, and everything else flat.

    On the -0.3 shot there is a blip in the bottom right corner of the Photoshop
    histogram that wasn't there in either of the camera's histogram displays
    (before or after the exposure). This blip turns into a significant double
    peak with each further step of exposure compensation. In the camera, this
    area of the histogram was dead flat. Any ideas what's going on?

    Thanks, Wilba
     
    Wilba, Mar 8, 2007
    #1
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  2. Wilba

    Paul Furman Guest

    That's the sky blowing out. I don't know why it isn't showing on the
    camera but if I set PS to show three color histograms, from left to
    right it's red, green, blue so your top three exposures are blowing out
    the blue off the chart. I guess the oly is only looking at an averaged
    greyscale. That's unfortunate. You can make a selection with the
    rectangular marquis to see the histogram of a portion of the image in PS
    and see the three humps when only the sky is highlighted.
     
    Paul Furman, Mar 8, 2007
    #2
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  3. Wilba

    Stan Beck Guest

    Wilba,

    Different exposures will generate different histograms. The 0 composition
    looks good. When you under expose, you will be shifting all of your data to
    the left, so the extreme light areas (right) might be more spread out. Visa
    Versa.

    Also, histograms from the camera won't always match Photoshop, or a
    different brand camera for that matter. It is just a guide, to visually see
    your value distribution, and to see if you are clipping your whites or
    blacks.

    Look at your 0 comp exposure - if your ends look like this, you are OK. You
    might still need to use your Shadow/Highlight filters to pull detail out of
    your shadows, though.

    --
    I really hate to eat on an empty stomach.

    Stan Beck > From New Orleans to Brandon MS
    To reply, remove 101 from address.
    ***
     
    Stan Beck, Mar 8, 2007
    #3
  4. Wilba

    Paul Furman Guest

    That might not have been clear.
    Each of those peaks is a different color.
     
    Paul Furman, Mar 8, 2007
    #4
  5. Wilba

    Wilba Guest

    That's how I understand it.
    Yep, that's how I think of it. In my experience, there is a reasonable match
    between the shape of the histogram in the camera and in PS, so it comes as a
    shock to me that there is a significant isolated peak appearing in one that
    didn't show in the other.
    Yeah, that's my basic understanding, so now I'm working on refining that,
    taking into account the advice to "expose to the right of the histogram". At
    this stage I'm not a sophisticated user of PS (and I have the cut-down
    version), so it's a multi-facetted task. :)
    Yeah, that's a significant part of the learning.

    Thanks!
     
    Wilba, Mar 8, 2007
    #5
  6. Wilba

    Wilba Guest

    "Top three" on the page - 0.0, +0.3, and -0.3?
    By a different method or alogithm to the RGB in PS .... Would you tell me
    more about how you see that explaining the anomaly? I get some idea of what
    you mean, but I can't quite relate it to the anomaly.
    I see what you mean about the three separate red, green, and blue peaks.
    Thanks a lot.
     
    Wilba, Mar 8, 2007
    #6
  7. Wilba

    Paul Furman Guest

    Yes, only the bottom -0.7 & -1.0 are 'un-blown'.
    All I can think is that the camera histogram does not count peaks unless
    all three colors are overlapped. The sky is strongly blue so there is a
    gap before the green registers and another gap before the red in the sky
    shows. You wouldn't think there would be *any* red but you can't get to
    white without blowing the red too. Once the green blows out, the sky
    will look hazy muddy brownish-red.

    I guess you will have to rely on just eyeballing the color on the LCD.
    What happens when the first (blue) channel blows in the sky... is you
    get a greenish-white sky that cannot be recovered to blue. So all I can
    recommend is to judge the color if it looks too green-white.

    One thing you can do in extreme lighting is take two exposures, paste
    them together in photoshop & use a very large soft edged eraser to
    reveal the deep blue sky below. I actually do this pretty often.

    But in your examples, the darkest -1.0 exposure looks fine, I don't see
    any need to lighten the shadows.
    You are welcome :) Nice job of testing. Groovy windmills.
     
    Paul Furman, Mar 8, 2007
    #7
  8. Wilba

    Wilba Guest

    Yeah, I can see that using the different colour channels in PS. But the
    histograms are quite different for these resised images than it was in the
    originals, so I'm not sure how valid it is to draw conclusions from them.
    Ah, OK, that makes sense to me.
    That advice works for me, thanks.

    I've been trying to emulate "blowing out a channel" in PS (Elements) but I
    can't work it out to make an image look like you describe. How would you do
    it?
    Yeah, I've come across this technique a lot lately. I'm not the kind of
    photographer these days that uses a tripod very often, so it probably won't
    happen much, but it's good to know how to do that if I want to take the
    trouble. :)
    Thanks. I'll do it again with smaller images so that resizing doesn't
    confound anything.
    They sure are. :) They power my PC and charge my camera batteries -
    http://www.bcse.org.au/default.asp?id=70&articleid=47.
     
    Wilba, Mar 9, 2007
    #8
  9. Wilba

    Paul Furman Guest

    I'm not sure. Levels works a little different, and curves, moving the
    middle around, not the whole thing. I use RAW files & the exposure
    slider for that. Brightness possibly.
    Set the top layer to 50% opacity, hold down the control key, slide it
    roughtly into position & with the control key still down, use the arrow
    keys to refine, or control-T (transform) if necessary, control-dragging
    corners & rotating as needed. A bit more fancy is to set the layer mode
    to 'Difference' then you can really see when it locks into place.
     
    Paul Furman, Mar 9, 2007
    #9
  10. Wilba

    Wilba Guest

    The main difference is that they have a big blob of solid black in the top
    left corner. :) The histogram is broadly the same after just resizing an
    image, but I can see some differences in the detail.
     
    Wilba, Mar 9, 2007
    #10
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