Everyone has a stockpile of images that are just too dark. The flash didn’t reach far enough, or perhaps a bright background caused the camera to underexpose the subject. Thankfully, you can repair some of the damage on your PC.
The Histogram Is Your Friend
The most precise and powerful way of correcting the exposure in a photo is by using your image editor’s histogram control. That’s good news, because once you understand the point of a histogram, you’ll find it’s really easy to use.
You can also avoid exposure problems by using your digital camera’s histogram display, which I discussed a couple of years back.
Histograms might look intimidating (they seem to remind people of high-school math class), but they actually aren’t that complicated. A histogram is a graph that shows the relative amount of information stored in each color channel; in other words, it displays how many pixels are dark and light in your image. The left side of the graph represents the darkest part (shadow) of the image, while the right side is the lightest (highlights). The central area represents mid-tones.
Here’s the most important thing to know about a histogram: You want the information distributed across the full range of the graph, not bunched up at the extreme edges. If there’s a spike of pixels at the extreme left or right, it indicates the photo is underexposed (on the left) or overexposed (on the right).
You can’t fix a truly under- or overexposed photo, because those pixels recorded pure black or pure white. Fiddling with the histogram can’t extract any detail or color information from them; all you can do is turn those pixels varying shades of gray.
As long you don’t see deadly spikes of under- or -overexposure in a photo, you can’t characterize an image as “good” or “bad” (or well-exposed or improperly exposed) just by observing how the pixels are distributed across a histogram. If a photo has a big “hump” towards the dark end of the graph, that doesn’t necessarily mean it’s underexposed, for example. The picture might have a dark subject, and you might very well like it that way.
Adjusting Light Levels
Most good photo editing programs have a handy tool for adjusting the light levels in your image using the histogram and a few sliders. Let’s look at how to correct an image using the Levels control in Adobe Photoshop Elements.
To begin, we want to see a photo’s histogram. If you’d like, open this sample photo–note that it’s kind of dark and seemingly underexposed. Choose Enhance, Adjust Lighting, Levels to open the Levels dialog box, which shows the histogram.
The photo above has a histogram like the one to the right. You can see that there’s a big flat region on the right side of the graph, where there are few bright pixels. To improve the exposure using the Levels tool, simply move the sliders under the histogram to set the white and black points, which stretches the distribution of brightness information in the image. Drag the white-point slider (the one on the right edge of the graph) to the left to meet the point where the graph flattens out. As you drag the slider, you’ll see the photo brighten.
If necessary, you could also drag the black point slider to the left–that’ll darken parts of the photo–but there’s no need to do that in this case, since the photo has good tonal range in the shadows. You could try to fine-tune the mid-tones in the photo by moving the gamma slider to the left or right. An adjusted histogram might look something like the one in the screen shot to the left, in which the white-point slider has been moved far to the left to brighten the image overall and the gamma slider has been moved slightly to the left to enhance the mid-tones.
It’s really important to eyeball the photo as you adjust the sliders–move the sliders a little at a time and stop when you like the result. You might only drag the white-point slider a small distance and decide the photo looks good with that minor adjustment–as the final version of our wolf photo shows. It’s all up to you. My advice: Load some of your favorite photos into your image editor and experiment.
Hot Pic of the Week
Get published, get famous! Each week, we select our favorite reader-submitted photo based on creativity, originality, and technique. Every month, the best of the weekly winners gets a prize valued at between $15 and $50.
Here’s how to enter: Send us your photograph in JPEG format, at a resolution no higher than 640 by 480 pixels. Entries at higher resolutions will be immediately disqualified. If necessary, use an image editing program to reduce the file size of your image before e-mailing it to us. Include the title of your photo along with a short description and how you photographed it. Don’t forget to send your name, e-mail address, and postal address. Before entering, please read the full description of the contest rules and regulations.
This week’s Hot Pic: “Sky Bridge,” by Matt Petersen, Aloha, Oregon.
Matt says: “I took this picture with my Canon Rebel XT on a bridge tour around Portland. The sun was hiding in the clouds, but it gave a very interesting effect to the shot. I did some light cleaning and converted it to black and white using Picasa.”
This Week’s runner-up: “Beach Seating,” by Joyce Sorensen, Clear Lake, Indiana.
Joyce says: “I took this picture while walking on the beach at Treasure Island, Florida, in January. It just looked as though this egret was contemplating whether or not it should take a seat on this bench.”
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