Digital Focus: Punch Up Photos With the Histogram

Feature: Liven Up Your Photos With the Histogram

What does it take to make a good photograph? Before you start talking about composition, the rule of thirds, kittens hanging from tree branches, or sunsets, consider this basic element: proper exposure. A well-exposed image has just the right amount of light in the scene to properly illuminate the subject, so that images are neither too dark or too light. A good picture has a good exposure "spread" over the total range of pixels in the image.

The best tool to use for correcting exposure is the histogram, a graph that shows how many pixels are dark and light in an image.

Interpreting the Histogram

Where can you see a histogram? Well, many digital cameras have a histogram mode; take a picture, and you can immediately see a histogram to assess its quality. In last week's newsletter, in fact, a reader's question referenced a friend who uses the histogram to discard badly exposed pictures without even looking at them.

I wouldn't go that far, but I will validate the notion that a camera's histogram can give you a very good idea of whether a photo will look good on a computer monitor and in print.

The histogram on a camera is really just for reference. Your image editing software probably also has a histogram display--and we can use that one to substantially improve your photos. Take a look at Paint Shop Pro histograms for a couple of run-of-the-mill images: coins and dancers.

The left side of the graph represents the darkest part of the image, while the right side is the lightest. The vertical axis shows how many pixels are in each of the image's brightness levels. A graph like one for the image of coins has a lot of midtones, while the dancing girls image is mostly filled with dark pixels--it's underexposed. We can use that information to tweak the brightness and contrast.

Using a Histogram Adjustment

I use the histogram to adjust many of my pictures, and I suggest you try the same. It takes only a few seconds and can improve almost any shot. Most good image editing programs--like Paint Shop Pro, PhotoShop, and Photoshop Elements--have a handy tool for adjusting the light levels in your image using the histogram chart and a few sliders. In Paint Shop Pro, it's found by choosing Adjust, Brightness and Contrast, Histogram Adjustment. Adobe Photoshop Elements, on the other hand, puts this control in Enhance, Brightness/Contrast, Levels.

Regardless of which program you use, the tool works more or less the same way. Just move the triangle sliders under the histogram to set the white and black points, stretching and optimizing the distribution of brightness information in the image.

Suppose you have an image like one I took at a play. Open the image in Paint Shop Pro and choose Adjust, Brightness and Contrast, Histogram Adjustment. Since the curve of the graph drops off before it reaches the right side, that tells us there are very bright few pixels. To fix this, drag the white triangle under the graph to the left to meet the point where the graph ends. In other words, you want to stretch the graph to take advantage of all the available space. That sets this point in the image as white, and should brighten the image. You should see the graph stretch as you drag the end points. Close the adjustment tool, and you'll see an image that has better exposure.

In your own images, be sure to set both the white and black points in the histogram, if necessary. Finally, use the gamma slider--the gray triangle--to adjust the overall brightness level in the image's midtones. That's all there is to it--and those 30 seconds just gave your photo more punch.

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