Understanding the Value of the Histogram
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Thread: Understanding the Value of the Histogram

  1. #1
    Buddysmom's Avatar
    Buddysmom is offline Senior Member
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    DefaultUnderstanding the Value of the Histogram

    If you understand what histograms are and how to interpret them, they can be very useful tools. First of all, what is a histogram? A histogram is a graph of every pixel of color in a photo. Histograms break down each pixel in a photo and assign it a value from 0 (completely black) on the left side to 255 (completely white) on the right side. Reading from top to bottom on the graph, the height of the peaks tell you have many of each color pixel is present. There are no correct or incorrect histograms. Histograms just are. They just tell you how many pixel colors there are and what their distribution is. Most histograms are black on white. They turn everything into shades of black, white and gray. In some programs you can view histograms broken down into red, blue and green.

    So if histograms just are, how do you use them to tell you if a picture is well exposed or not? First and foremost, you have to know what your picture is supposed to look like. You have to know in your mind's eye how much of the photo is dark, light and in between. You have to know what to expect from your histogram. Let's say I'm taking a picture of the full moon. What should I expect from a well exposed histogram? Think about it. What and how many colors should you see? Well, there should be a lot of dark sky. There should be a white moon with different amounts of gray in it. So what should our histogram looks like? A good moon shot histogram should show a large peak on the left side of the histogram. Because there is no detail in a black sky, the peak will probably go up the left side. There should then be a very thin line going toward the right with small blips along the way. On the right side, but NOT running up the right side, there should be a smaller peak that drops off as it approaches the right side. Why would the histogram look this way? It's because most of the photo is supposed to be black. There's a very small area that's white, and even less that's gray.

    Now let's say you're taking a picture of the beach, ocean, and sky on a bright sunny day. The well exposed histogram will show a large, broad beak in the center and gradually dropping off on both sides, but not running up either side. Why? It's because there is very little dark black or white white in the picture. The sky, sea and sand translate into a medium gray color. If there are a lot of white clouds in the sky, you may get a smaller peak near the right edge. If that smaller peak runs up the side, you're blown out (over exposed) the clouds. If when you check your histogram, the peak is too far left or right, you've under or over exposed the picture.

    If you're photographing a lone tree cover in ice and snow in a snowy field on a snowy day, you should see a large narrow peak on the right side of the histogram. It should not run up the side. If it does, you've lost detail in the white snow. When you look at the picture, there's very little dark or even medium colors in it. You would expect the histogram to be very one sided.

    In my mind, I break the histogram down into 5 sections. They are very dark, dark, medium, light, very light. I'm teaching myself to look at a scene before I take a picture and judge how many of the pixels should fall into each catagory. That is. How much of my shot is dark colors, medium colors, and light colors? I try to see the picture in black and white. Then when I look at the histogram, I have an idea what it should look like. If I'm expecting a lot of dark colors, and my histogram is shifted to the right, I know I've over exposed the shot and vica-a-versa.

    You cannot look at a histogram without seeing the picture and tell if it's a good histogram or not.

    I hope this helps you understand what a histogram is and what it isn't. If used correctly, it can tell you at a glance whether you should reshoot your photo or not. In an editing program, it can help you bring out portions of your photo that may be a little too dark or too light. As long as you haven't blown out any highlights, you can correct for overly dark or light areas with a good editing program if you know how to read a histogram.


    Eiderdowns That's My Buddy
    CDX, RE, WC, CGC, TDInc.
    Monnie

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  3. #2
    Marley is offline Member
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    DefaultRe: Understanding the Value of the Histogram

    awesome write-up. Very good and accurate info. I really like the part about seeing the image in black and white first before you take it.

    Some DSLR's have an option that will display the histrogram next to the image in "play" mode, and another that will show blown highlights or shadows by flashing them when you view on-camera.

  4. #3
    Buddysmom's Avatar
    Buddysmom is offline Senior Member
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    DefaultRe: Understanding the Value of the Histogram

    Yes, Canon dSLRs show the histogram on a screen next to the picture. I like the way Nikon camera display their histogram. They superimpose the histogram over the photo on the screen. I wish Canon did the same thing.


    Eiderdowns That's My Buddy
    CDX, RE, WC, CGC, TDInc.
    Monnie

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