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Photography Q&A
Histogram and Dynamic Range
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<blockquote data-quote="WayneF" data-source="post: 457212" data-attributes="member: 12496"><p>These statements are not correct. The histogram does not show magnitude.. period.</p><p></p><p>The histogram is a simple bar chart that shows the <strong>pixel count</strong> of all the brightness values from 0 to 255. The height is simply the count of pixels with that tonal value. Any peak is only there because there are a lot of pixels with that same value. The peak might be about dark or bright values.</p><p></p><p>If your image is mostly dark, there will be many dark pixels, so the the peaks will be at the low half.</p><p></p><p>If your image is pretty bright, there will be many bright pixels, so the peaks will be in the bright half.</p><p></p><p>There is no right answer about how it should look, since it depends on the scene in front of the camera. But dynamic range implies it should be fairly well filled, with some dark and some bright values. A very bright color should approach 255, but if the scene has no bright colors, then maybe not. The reason we pay attention is because anything past 255 is clipped, lost, not recoverable. A tall peak right at 255 is a count of many clipped pixels.</p><p></p><p>The vertical scale is always scaled, so the tallest peak will always be about full scale. Height has no absolute meaning, everything is relative. Because the histogram simply shows <strong>pixel count</strong> of each of the various tone values. It simply shows how the brightness range of your image is distributed, from dark to bright.</p><p></p><p>In the camera, there is one single gray histogram (technically called luminance, which is just a math model to represent gray scale values, and it cannot show clipping. It is NOT real data).</p><p></p><p>And there are also three individual Red and Green and Blue channel histograms, which do show the images real genuine non-math-manipulated true values. Ignore the single gray one, and for examining clipping purposes, pay attention only to the three RGB histograms. See <a href="http://www.scantips.com/lights/histograms.html" target="_blank">There are Two Different types of Histograms</a> for more.</p><p></p><p>And for what it's worth, the histogram is also showing the gamma encode values (the RGB data is gamma encoded). This means the middle gray midpoint is NOT at 128 mid-scale. It would be for linear data, but instead, the gamma value for that is about 187, or about 3/4 scale. However, this value is a bit vague, because the camera white balance and contrast and color profile setting shift it somewhat, in a variable way.</p></blockquote><p></p>
[QUOTE="WayneF, post: 457212, member: 12496"] These statements are not correct. The histogram does not show magnitude.. period. The histogram is a simple bar chart that shows the [B]pixel count[/B] of all the brightness values from 0 to 255. The height is simply the count of pixels with that tonal value. Any peak is only there because there are a lot of pixels with that same value. The peak might be about dark or bright values. If your image is mostly dark, there will be many dark pixels, so the the peaks will be at the low half. If your image is pretty bright, there will be many bright pixels, so the peaks will be in the bright half. There is no right answer about how it should look, since it depends on the scene in front of the camera. But dynamic range implies it should be fairly well filled, with some dark and some bright values. A very bright color should approach 255, but if the scene has no bright colors, then maybe not. The reason we pay attention is because anything past 255 is clipped, lost, not recoverable. A tall peak right at 255 is a count of many clipped pixels. The vertical scale is always scaled, so the tallest peak will always be about full scale. Height has no absolute meaning, everything is relative. Because the histogram simply shows [B]pixel count[/B] of each of the various tone values. It simply shows how the brightness range of your image is distributed, from dark to bright. In the camera, there is one single gray histogram (technically called luminance, which is just a math model to represent gray scale values, and it cannot show clipping. It is NOT real data). And there are also three individual Red and Green and Blue channel histograms, which do show the images real genuine non-math-manipulated true values. Ignore the single gray one, and for examining clipping purposes, pay attention only to the three RGB histograms. See [URL="http://www.scantips.com/lights/histograms.html"]There are Two Different types of Histograms[/URL] for more. And for what it's worth, the histogram is also showing the gamma encode values (the RGB data is gamma encoded). This means the middle gray midpoint is NOT at 128 mid-scale. It would be for linear data, but instead, the gamma value for that is about 187, or about 3/4 scale. However, this value is a bit vague, because the camera white balance and contrast and color profile setting shift it somewhat, in a variable way. [/QUOTE]
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