Understanding the Histogram

AB
Posted By
Andrew_Bokelman
Nov 14, 2003
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I am under the impression that the histogram is supposed to represent the tonal range. The distribution of darks and lights. But if I open an image and simply switch it between RGB, CMYK, and LAB, the histogram changes so dramatically it is as if it belongs to different images, even though there is no visual change in the image on the screen.

Can someone please explain.
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RK
Rob_Keijzer
Nov 14, 2003
Andrew,

You’re correct in your assumption on the histogram: it shows the distribution of pixels as a function of lightness.

This means that on the left side you see the darkest pixels that are possible in the current colour model, and on the right side the lightest possible.

These colour modes (RGB, CMYK and Lab) have different Gamuts. That is colour range.

You’ll agree that (on paper) there in nothing blacker than black ink, so in CMYK a pixel with this property is bunched up the left side of the histogram.

In Lab, however, the same pixel is still the same shade of black but the colour range of Lab is capable of having tones that are more black.
So this particular pixel will show up a bit to the right.

When you discovered the differences in histograms you’ve probably noticed that the histograms went smaller when you changed from CMYK to RGB to Lab. (the latter having the widest range of the three).

Hope this helps
Rob
DM
dave_milbut
Nov 14, 2003
digging a little deeper, why should we use the historgram (including the new historgram palette in cs)? what does it buy the average user? can it be used to manipulate images? or is it just a visual guide to what your changes will do to the overall tonal range of the image?

I mean, if you apply an adjustment with preview already turned you’re getting visual feedback directly on your image as you tweak the adjustment’s paramaters. What does also using the historgram (and it’s new palette) get you (or me <g>)?

lisening to my betters with interest,
thanks, dave
RK
Rob_Keijzer
Nov 14, 2003
Dave,

Not being better at all, but I know what a histogram can show right after I took a photo: It can show me that I failed (and why).

In Photoshop, when leveling and curving etc. I often wonder if I’m getting blind: Is that sky blowing out white or is my eye fading?

In the Histo I can see what I’m doing.

Rob
GS
Gustavo_Sanchez
Nov 14, 2003
I’d read this < http://luminous-landscape.com/tutorials/understanding-series /understanding-histograms.shtml> and this < http://www.marginalsoftware.com/HowtoScan/tutorial_page_1.ht m>. It might help.
AB
Andrew_Bokelman
Nov 14, 2003
These colour modes (RGB, CMYK and Lab) have different Gamuts. That is colour range.

Rob, take a few pictures and switch between the modes. See if it looks like simply a gamut change. LAB is the most extreme change, with most of the pixels being bunched in the center in quantities that reach the top of the histogram, but still showing a tiny amount of pixels that reach the ends.
JS
John_Slate
Nov 14, 2003
And a full histogram means what?

You could have an absolutely stunning high-key photograph in which the darkest pixel would be called a midtone in most shots, and the lopsided histogram would not detract from the austerity of the image.
JM
John_Mensinger
Nov 14, 2003
A histogram is simply a graph of tonal ranges (pixel luminosity), contained in an image. There really isn’t a "good histogram = good image" convention, (though changes in a histogram could illustrate ‘damage’ or improvement in an image through editing).

A histogram is most useful as it relates to image capture, (scanning or digital camera), in that good scanning software and good cameras offer histogram previews that can be used to measure/adjust/confirm that all available tonal range in a given image has been captured within the density range possible with the device.
MV
Mathias_Vejerslev
Nov 14, 2003
I use the Histogram with my images from the very first step, to check for channel clipping (saturated colors) and contrast. Then I work my RAW files to expand or contract the histogram (with image preview ofcourse) to contain all the values to get a proper exposure.

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