Reading histograms

Discussion in 'General Technical Discussion' started by Iliah, Apr 6, 2007.

  1. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    This shot courtesy Bill Janes is the back LCD of D200; the subject is T4110 transmissive Stouffer grey step wedge composed of equally spread 41 steps, density step = 0.10 (1/3eV), maximum density = 4.05, more about the product here: http://www.stouffer.net/TransPage.htm#transmission step

    The whitest part of the wedge corresponds to the second (massive) peak from the right. The rightmost small peak is due to light leak.

    [​IMG]

    This is not a scientific test, but a demonstration how Nikon calibrate their default tone curve in cameras; it also allows to match histogram with exposure compensation (for green channel only of course, if you do not use UniWB).
     
  2. TomaS

    TomaS

    497
    Aug 3, 2006
    Corrales, NM
    I have no clue what this means, let alone how to use the info. Can you please explain what this is telling us mere mortals?
     
  3. Yes yes...please do.:confused:
     
  4. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    This means that first three rectangles cover about 5 eV, and if, for example, your highlights are in the middle of the first rectangle from the right, you have to increase exposure by about 1 stop.
     
  5. Dear Iliah,
    Please correct me if I'm wrong. From the histogram of the wedge, the first rectangle from the right covers a bit more than 1 1/3 stops, the second one about 1 1/2 stops, the 3rd one 1 2/3 stop and the last one about 2 1/3 stops.

    If the first rectangle covers 1 1/3 stops and for example, the highlights are in its middle, the EV compensation must be 2/3 stop, not 1 stop. Am I missing something?

    Best regards.
     
  6. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Dao Dang,

    The scale is not a calibrated one, if you make a little more controlled experiment then you will see that with contrast set to "normal" each of first three rectangles from the right cover 1 2/3 stops, 5 stops in total. The last rectangle contains the rest of the dynamic range, exact amount depending on camera model/ISO/WB/noise. For jpgs, from my experience, compensation from the centre of the first rectangle to the right is close to 1 stop (5/6eV, but we can't set it that way in camera). I do not mind tiny specks in the image to go pure white. Tiny white specs will disapper on the print because of ink smearing and dot gain. Because raw has more headroom centre of the first rectangle allows me to increase exposure by 1.5 to 2 stops, depending on the raw converter mode.
     
  7. OK, please correct me if what I say here is wrong, as I'm attempting to move it to practical shooting ( I chimp my shots to help me with exposure):

    My meter will generally bias the exposure to the left (or under expose) and I actually have much more room to open the exposure on the right. For example, when taking my ski racing pics, I expose for the snow then open up two stops (a la Thom Hogan). After the first few test shots, I tweak this advice up or down slightly.

    What this test shows, I think, is that, I may be doing this without necessarily knowing what I'm doing (ain't life grand). My non-scientific observation is that CS2 and other programs in auto mode like to adjust exposure about 1/3 of stop.

    Or am I still all thumbs?
     
  8. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Spot-/centre-metering from snow and compensating plus 2-3eV is such an old knowledge I won't associate Thom's name with it. :) I use 2.5 eV, it works with our converter well enough.

    By default ACR interprets exposure differently from my practice. To see that and to match what you see on Nikon's in-camera histogram you can shoot some grey step wedge.
     
  9. OK Iliah, thanks for the response. Since I didn't know the original source, I was trying to establish that the exposure guideline was not created by me. So will let it be...

    Know, your second comment....

    I agree with ACR interpreting exposure differently, I thought it was me.

    I don't understand your comment about the grey step wedge. Even though I've taken 12K worth of photos digitilly, in your response, don't assume I know what I'm doing.
     
  10. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Grey step wedges are important tonality control tools; they also help to adjust white balance linearly across the whole tonal range, from highlights to shadows. For a list of available grey step wedges, see here:
    http://imatest.com/docs/q13.html

    For everyday use I have Kodak/Tiffen Q13, Jessops, and Stouffer charts. Those charts have known densities of grey, and flat colour response. When a shot of such a chart is opened in processing software it is easy to determine if the processing achieved correct dynamic range and contrast/tonal response. Charts are also useful to correct for any colour/light/lens/processing inconsistencies,.
     
  11. Dear Iliah,

    Thank a lot for the response. These informations will surely help me in my learning of correct exposures.
     
  12. Thank you Iliah, very useful information.
     
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