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Understanding Histograms

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Histograms are easy. It's simply a bar graph that shows the range of brightness available in the scene, from black at the extreeme left to white at the extreme right. The higher any area of the graph, the more of that level of brightness that exists in the picture. What you see at a glance is that there is a fair amount of black, a lot of mid-tones and only a small amount of highlight tones.

It takes a little while to learn how to determine what to do with this information, but it's a vital tool when working in image processing software and can be equally indispendable when working with a digital camera.
Michael

 

By Howard Smith (Hssmith) on Wednesday, March 13, 2002 - 04:54 pm:

Thanks Michael. I now realize I did know what a histogram is and what it is saying. Unhappily, I haven't the foggiest notion of what to do with it. The example you provided shows a histogram, but just looking at the image I can see there is a lot of black and mid-ish tones and not much highlite. I do some rather mechanical things when I scan a chrome that seem to give good results, but I don't truly know why. Any help on the horizon?

 

By Ebert Steele (Steele) on Wednesday, March 13, 2002 - 09:12 pm:

Thanks Michael,
I understand what a histogram is. It is a plot of the distribution of variables that occur within an event - in this case, light values from 0 to 255 within an exposure. What I don't understand is how by looking at it I can judge an exposure.

A low key image, for example would show a large distribution low values at the low end of the graph and the opposite for a high key image and both be could be properly exposed. But what value is that to me in making decisions about 1/2 stop (or even 1 stop) of more or less exposure. In your example image above if 1 stop of exposure was added I could envision somewhat how the histogram would change but if I were to look at that in isolation (without its corresponding image) what would that tell me about exposure?

This is not meant to be argumentative, but rather to show my confusion. Did I do good? :)

Maybe it will come to me, but right now when I look at a histogram its interesting but of little value to me. I have so much to learn that I have set some priorities. Color management is the hill I'm trying to climb right now and in the meantime with exposure I still have my Pentax 1 degree spot meter to fall back on.

Steele

 

By Michael Reichmann (Mreichmann) on Wednesday, March 13, 2002 - 10:06 pm:

There are several ways that viewing a histogram can be useful, as well as simply being of interest.

By way of example, if you see the values bunched up at one end or the other you can change the exposure to move them more centrally, thereby expanding the dynamic range available. You could make this determination with a spot meter, but seeing it graphed allows for instant recognition.

Michael

By Zlatko Batistich (Zlatko) on Thursday, March 14, 2002 -

03:50 pm:

Ebert -- when you look at a good histogram, with all of the values nicely arranged, its usefulness may seem questionable. But when you see a bad histogram, with highlights or shadows being cut off at the ends, you instantly know what to do and recognize how useful it is. You then "move" the exposure up or down until all of the information you want to capture is within the recording range. With a low contrast scene, this is easy. But with a high contrast scene, the histogram helps you easily decide how to capture the greatest range of information and how to lose information only where it is least needed for that particular image. A histogram can be extremely useful, provided that its range closely matches the recording range of your medium (whether film or digital).

http://www.marginalsoftware.com/

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Concepts: Histogram, Debut albums, High dynamic range imaging, Bar chart, Function, Value, Exposure, Meantime

Entities: Pentax, image processing, confusion, Michael Reichmann, Ebert Steele, Michael, Ebert

Tags: histogram, image, show, spot meter, contrast scene, recording range, key image, bad histogram, information, image processing software, good histogram, distribution low values, low contrast scene, degree spot meter, low key image, high key image, high contrast scene, useful, bar graph, extreme right, mid-ish tones, highlight tones, Ebert Steele, light values, vital tool, mechanical things, corresponding image, example image, digital camera, good results, dynamic range, low end, little value, instant recognition, particular image, greatest range, Color management, Michael, argumentative, questionable