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11-28-2014, 11:03 AM   #1
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histogram question

I pretty much understand (and rely upon) the camera's histogram, but I have a question..
If the Y Axis is to represent the number of pixels in that tonal range, what does it mean with the Y Axis is very flat but spread out across the X Axis and neither end is clipped at 0 or 255 ?

11-28-2014, 11:43 AM   #2
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That would suggest that the number of lighter, darker, and neutrally-lit pixels is more or less equal.

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11-28-2014, 03:33 PM   #3
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Adam - contrast that with a graph where the Y Axis rises very quickly from 0 through the highlight section filling the Y Axis to 255.. still no clipping but the graph seems to be filled (# of pixels)
11-28-2014, 06:13 PM   #4
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QuoteOriginally posted by dubsider Quote
If the Y Axis is to represent the number of pixels in that tonal range,
I use the histograms a lot and sort of understand them.
But if the y axis represents the number of pixels then wouldn't the area under the line be always equal?
Glancing at my camera that would not seem to be the case although it is hard to tell. And certainly doing the curves in gimp the area changes.
May be the scale changes on the y axis to fit things in.?

12-02-2014, 02:49 PM   #5
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QuoteOriginally posted by GUB Quote
I use the histograms a lot and sort of understand them.
But if the y axis represents the number of pixels then wouldn't the area under the line be always equal?
Glancing at my camera that would not seem to be the case although it is hard to tell. And certainly doing the curves in gimp the area changes.
May be the scale changes on the y axis to fit things in.?
I think this is the key. Some software, and even cameras, automatically scale the y-axis to fit things in to some extent.

As an example, when you are clipping an image, the end points of the histogram will be very high leaving the remaining portion of the histogram looking flat. As a result it's all relative.

In reality, I think you really only have to know it's relative and understand what the shape of the histogram means roughly and make sense of it.

For instance, the earlier post about a histogram that was essentially flat. Did this make sense for what the image looked like? When you end up with spikes at the ends when things are clipped, does that make sense? When you see a more typical curved histogram, where is the peak?

To me, the y-axis numbers for a histogram and the general shape of the histogram mostly serves as a qualitative guide. On the camera it helps you to readjust and reshoot if the exposure is too high or too low. In PP it helps for guiding adjustment of the brightness, contrast, curves, etc. I almost never look at specific numbers or worry about them.
12-03-2014, 07:21 AM   #6
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They definitely scale the y-axis. In the Gimp or Photoshop, you can see the pixel count for each of the brightness levels. Take an image with a big white area that makes a spike to the right. Check the pixel count at a couple of mid-tone levels then crop out the white area. The pixel count of the midtones won't have changed, but the bars representing these values will be taller if you've cut out the area that caused the spike.

A flat histogram just means all brightness levels are equally represented (or nearly so). You can 'cheat' one of these into existence by taking a picture of a smooth gradient that ranges from white to black, or artificially make one in photoshop.
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