Histogram.....height means....?

<p>I get the histogram left to right info, from black, to white, from 0 to 255, but what does the the height of the various aspects of the histogram mean please?</p>

<p>The amount there is of the varying black to white 'stuff' in the picture, at any particular point across the 256 that there are between 0-255. So left to right its tone, top to bottom its amount. Someone else may put it more eloquently.</p>

<p>So for example if my mid tone 'hump' is braking way through the top of the histogram that means.....?</p>

<p>Hi Tony, the hight of the histogram graph (more appropriately called the Y-axis) represents luminous intensity at any given point of the horizontal axis, represented as amplitude once photons are converted to electrons.</p>

<p>The top of a full histogram graph represents signal saturation (or clipping) which one avoids through camera settings. The bottom of the graph represents an absence of light.</p>

<p>The main thing to watch for in the histogram is huge spikes at the left or right edges, which are likely to indicate blocked shadows or blown highlights respectively. What happens in the middle is generally less interesting.</p>

<p>Also, if your camera has an option to display a three-color histogram (which is basically separate histograms for the red, green, and blue channels), use that instead of the one-color luminance histogram, and check all three channels for left and right spikes. This will generally be more accurate than the luminance histogram.</p>

<p>Thanks for the responses, but its not sinking in. :o(<br>

For ease of explanation lets call the left to right range 0-255. If at around 200, I have a hump that goes way off the top of the histogram, this means what..... Surely its not indicating that anything is clipping exposure wise as that would then extend right along the scale to (and beyound) 255. If my image is fairly high key, or I'm shooting packs on a cream BG, then would this be acceptable to expect an overload at the 200 mark?<br>

Hope thats making sense</p>

 

<p>If you have a huge spike at the horizontal position corresponding to luminance value 200, all it means is that you have a lot of pixels with that luminance value. But you probably don't care about that, since 200 doesn't indicate a problem with blocked shadows (which would be down around zero) or blown highlights (up around 255).</p>

<p>Let's say you take a picture of an 18% gray card, zooming in close enough that the whole frame is filled by the card. With a nominally correct exposure, this should give you a histogram with a thin, tall spike right around luminance value 128. There should be nothing, or very nearly nothing, at the left and right sides.</p>

<p>Tony,</p>

<p>The height represents the frequency or number of instances in the photo that are at that intensity. Let's say the photo was completely black; There would be a bar up above the 0 on the x axis, and nothing anywhere else on the histogram. If each light intensity from 0-255 were equally represented, you would have a flat line. If 2/3 of the scene were at one intensity and the other 1/3 at another single intensity you would have two lines, one twice as high as the other.</p>

<p>Here's an article that might help as well. <a href="http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml">Understanding Histograms</a></p>

<p>Inadvertant double post, sorry.</p>

<p><a href="http://en.wikipedia.org/wiki/Image_histogram">http://en.wikipedia.org/wiki/Image_histogram</a><br>

<a href="http://super.nova.org/DPR/Histogram/">http://super.nova.org/DPR/Histogram/</a><br>

If the vertical goes off scale, is it just scaling of the LCD display that's not adjustable like a graph in Excel?<br>

Jim</p>

<p>Thanks Craig, got it. </p>

<p>You need to check out these links too:</p>

<ul>

<li><a href="http://www.cambridgeincolour.com/tutorials/histograms1.htm">Camera Histograms: Tones and Contrast</a></li>

<li><a href="http://www.cambridgeincolour.com/tutorials/histograms2.htm">Camera Histograms: Luminance and Colour</a></li>

</ul>

<p>Thank you, I've previously read through all of those but still could't grasp why the vertical goes beyond the ceiling point of the graph, to it, yes, but not beyound it.</p>

<p>But I've concluded that for my work, its doesn't really matter. I can view the RAW files 'live' and judge clipping on the histogram left to right.... good enough for my needs</p>

<p>thanks all.</p>

<p>Hi Tony,</p>

<p>The vertical scale on the histogram is the number of pixels in the image that have each particular value (0 to 255 in each of the three channels).</p>

<p>For example, if you fill the frame with a shot of a piece of bright red plastic that's truly monochromatic, i.e. pure red, your histogram would have one huge spike around the middle, in the red channel. The remainder of the histogram, including the entire green and blue channels, would be zero.</p>

<p>The vertical axis is scaled so you get more information in the lower values. If they set the top of the chart to the maximum possible count (10 million for a 10Mp camera), everything else would be down along the bottom, with no visible changes. So they set the top to a value that's a reasonable maximum for average subjects.</p>

<p>- Leigh</p>

 

<p>An histogram is nothing more than a count of pixels at each luminosity on a 0 to 255 scale. Colors are counted separately or summed for the RGB composite. A statistician would plot the original count, scaled so that the highest peak is not clipped, or normalized as a percentage of the total count. Photoshop does neither, since it is common to see peaks truncated, so the vertical scale is meaningless except for comparison within the same chart.</p>

<p>as long as it's not on the left and right "walls" of the histogram....don't worry about it....means nothing but statistics....no impact to image quality that I've ever seen</p>