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Using Histograms
Updated  Wednesday, August 09, 2000
by Randy Glass - Copyright 2000 by the Feather River Canyon News - All rights reserved.
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Reading and Using Histograms
[NOTE: This lesson is written for Photoshop and Photoshop LE. Other applications may have similar adjustments. See the documentation of the application or check the help file. Even if your application does not work this way, the lesson will help you understand Histograms and how to interpret them.]

      Histograms have nothing to do with pictures of female reproductive sterilization. A histogram is a graph that shows the “digital imager” the balance and placement of the pixels in an image. Below is a portion of the histogram window in Photoshop of a properly exposed image. Let's take a look:

      As you can see, this image features a sort of bar graph with lots and lots of very narrow, vertical bars (lines, really) all blended together horizontally. There are 256 bars numbered from zero on the left to 255 on the right. The bars at the left represent the darkest pixels and the bars at the right represent the lightest pixels. Each bar is a level of luminosity (basically, brightness) representing the 256 levels of grayscale that the human eye is generally given credit for being able to recognize. The taller a bar the more pixels there are in the image at that level. In the example above there are some peaks and valleys, and that is to be expected. What is good is that the graph is fairly well represented all the way across. We would expect the image represented here to be fairly well exposed.

      There is some other information in the window that gives us some numeric data. The count is interesting as it tells us how many pixels there is at the specific level where the cursor is located, which in this case is at level 128, or about the mid-range of the tonal scale (about half way between 0 and 255). Moving the cursor right and left along the histogram will reveal the number of pixels at that level.

      Let's now take a look at the three sample images we will be using for this lesson:

Underexposed Image
Correctly Exposed Image
Overexposed Image
      As you can see, the center image looks fairly decent and the other two, for different reasons, are quite poor. Many of today’s better digital cameras have a histogram function built-in. It doesn’t allow for any adjusting of the image once it has been captured, but when you know how to read histograms they allow you to learn whether the image you just captured was correctly exposed.

     Many cameras report the histogram of an image right in the camera, superimposed over the image right on the LCD screen. With that in mind, let's take a look at what my camera reported when I captured the three images above:

       In this image, the histogram is devoid of any data on the right side of the graph. This shows that the image is lacking in data in the light region and is quite dark overall. It has been underexposed.

       In this image the histogram is balanced well to the right. You can see the dramatic spike in the graph on the right side showing an overabundance of white. If an image has a white wall in the background and you used your flash that might be expected, but this image is just way overexposed.

       As you can see from this histogram there is a fairly decent balance of levels. The histogram shows activity all across the range showing that the image was fairly well exposed. Although the histogram looks a little heavy in the dark range, you would expect that in a shot with lots of leaves in deep shadows such as this. There is little in the white area at the right side of the histogram, but that's all right. We wouldn't expect to see much pure white in a landscape shot.

      So, when the histogram shows us that we have captured a poor image, should it be deleted? In Photoshop you have the ability to adjust the image using histogram information to better reflect reality. It is located under “Image/Adjust/Levels.” I opened the original underexposed image shown above in Photoshop and examined its histogram:

       As you can see, the histogram's balance and levels appear to be a bit different from what we saw on the camera, but overall it reflects the same thing- an underexposed image. What we are concerned with here are the three little triangles under the histogram itself. They can be used to re-balance the histogram of the image. (The cursor is pointing to one of them.)

      Here’s the histogram with the sliders moved to create a more pleasant image. I first moved the far right slider towards the left. This is the White point. You are telling the application where to set the level for pure white. I moved it to where there seemed to be a beginning of the image. I played around with it in this area until it looked right.

      Next I moved the left slider to the right a little. This sets the black point. You are assigning the level for pure black. In this image the black point was quite sensitive and needed only a slight amount of adjustment.

      Finally I moved the center slider to reach a visual balance. This sets the mid-tone or center point for the image. This one is a bit more flexible and where to set it depends on the individual image.

      For all these settings simply play around with them. Move them to extremes and you will quickly get a very good idea of what works and what doesn’t. It all might sound a bit complicated but it really isn’t. The nice thing about Photoshop is that these adjustments are interactive. You can check the “Preview” box and watch the opened image as you move the sliders. The changes are shown in the image so you will know whether you are doing the right thing immediately. So you have seen how to move the adjustments about, but how well did the above adjustments work on this image? Let’s take a look:
This is the underexposed image from the camera This is the same image after it had been adjusted using the Levels adjustment in Photoshop Here is the properly exposed image from the camera as a comparison
You can see that I have successfully rescued an image that otherwise would have been lost

      Don’t get too excited. You will find some images beyond help. The overexposed image of the three was beyond help. So much of the image had been “blown out” (overexposed to the point of turning to almost pure white and losing detail) that there was nothing left to adjust. In the same way, a dramatically underexposed image will also be lost. By the time you bring it back to the correct levels you will find lots of noise and no detail in the dark areas.

      When you check a histogram of an image in your camera, look for the same kind of things. If the histogram shows that the image is underexposed, open the aperture, use a slower shutter speed, set you external flash to a higher level of light, use some “+” EV adjustment, or combine some of those adjustments to get a properly balanced histogram. If the histogram shows that the image is overexposed use a smaller aperture, a faster shutter speed, turn down your flash (or turn it off), use some “-“ EV compensation or some combination to balance the histogram on the next image. The instant feedback we get from digital cameras is wonderful, but the added feature of the histogram makes the feedback dynamically useful.

      So there are no miracles here, but by using the histogram feature of your camera you can take better images, and using the histograms in Photoshop you can adjust your images like the pros! Now go out and create some great images!

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