Digital Focus: Use the Histogram to Avoid Exposure Issues

Feature: Use the Histogram to Avoid Exposure Issues

One of the most often overlooked goodies in your digital camera is its ability to display histograms. The function is frequently overlooked simply because it sounds intimidating. Histogram? That sounds like a cross between something you did in 10th-grade math class and some sort of uncomfortable medical procedure. How can that be useful?

A histogram display, which is included on most digital cameras, provides a quick indication of your picture's exposure quality. Is it overexposed? Underexposed? Just right? The histogram knows, and it's eager to tell you.

Finding the Histogram on Your Camera

As I say, most digital cameras these days include a histogram display. Check your user guide for details. Most cameras overlay the histogram on top of the picture after it has been taken--so you'll see it on playback. On many cameras, you can turn on the histogram overlay with the same control that lets you change the picture you're previewing on the screen. There are even a few models that will overlay the histogram as you preview the scene before you click the shutter release. That's the best option, since you can see even before you take the picture if it will be well exposed.

Understanding the Histogram

A histogram is a graph that displays how light is distributed in your picture. The left side of the graph represents the shadows, while the highlights are on the right. Here's what that means: If the histogram has a high peak on the left, you can deduce that a lot of pixels in the picture are dark, or in shadow. A peak on the right of the graph means that a lot of pixels are bright, or in highlights. Peaks in the middle of the graph represent pixels in the midtones of your exposure.

And here's the real key to unlocking the power of a histogram: There should not be any peaks that get "cut off" at either end of the graph, as if they want to continue past the edge of the graph. When the histogram starts or ends with a peak that's already in the air, then you know that color information has been lost because the camera's exposure settings weren't correct for that picture.

So let's apply this knowledge to some real histograms. Take a look at this one; it's the sort of thing you're looking for. At the left side of the graph--the shadows--the graph starts at the zero point on the horizontal line and then curves upward. On the right side of the graph (the highlights) the little peak trails off to zero before reaching the far right edge of the graph. These shapes tell you that no part of the scene is over- or underexposed. The light is well distributed through the image. Overall, this shot should look good.

If you don't quite "get it" yet, that's okay--this histogram should help you understand the concept. In this underexposed picture's histogram, I see a spike in the shadows that starts "off the graph" to the left instead of rising from the horizontal axis. That tells me that this picture has lost data--perhaps quite a bit of it--in the shadows. Notice that there's also just a few pixels trailing off the right side of the graph, so a tiny bit of data might have been lost there as well. Ideally, the graph would blend into the horizontal axis before reaching the edge of the graph, but this looks pretty small and so isn't much cause for concern.

Here's a histogram of an overexposed photo. Here you can see a pronounced spike in the highlights. Don't worry about the fact that the height of the peaks is somewhat low in this picture; that's not an indication of under- or over-exposure. All you should really worry about is whether they breach the left or right edges of the histogram.

Finally, photos don't get much worse than what this histogram shows. There's very little information in the picture--it was taken in the shadows, without the benefit of a flash. So what do we have? Very little light information through the midtones at all. There's a deadly spike in the shadows at the left edge of the histogram, which guarantees that the picture is underexposed. Using auto-correction tools on this picture will promote a lot of digital noise, so the image isn't going to be very salvageable. But look at the highlight--there's a little spike there as well. This picture has a sunbeam in the middle of the scene, and that sunbeam has "blown out," or overexposed, a small portion of the image. I'd call it a total loss.

Some General Advice

So what should you look for when sizing up your shots on the camera? Obviously, you'd like to keep the histogram from spiking at either extreme end of the graph, where you'll lose data and have under- or overexposed parts of your picture. Also, note that the particular shape of the curve isn't all that important--it can be shallow, curvy, flat, or some combination. The overall shape really just represents the specific light distribution in your photo, and that's as unique as a fingerprint. Finally, you'll get your best results when the graph sits as far to the right as possible--but without clipping the highlights. Overexposure is always worse (and harder to correct) than underexposure.

Dave's Favorites: Cool Thumbnails With Photo Shaman

If you ever make thumbnail-sized versions of your digital photos--for a Web site, perhaps, or for photo album software--you will be very interested in a clever little program called Photo Shaman.

Photo Shaman, from Brave Orange Software, can take entire folders full of pictures and resize them down to any dimension you like. "Big deal," you say. I'd agree, if that's all it does--pretty much any decent image editing program can do that. But Photo Shaman goes much further: It can also embed your thumbnails inside a staggering array of miniature picture frames. There are all sorts of creative frames in Photo Shaman: colorful 3D frames, beveled frames, ragged edged frames designed from masks, frames with drop shadows, and more. I played with Photo Shaman for an entire afternoon without exhausting all the possibilities.

The program is powerful but simple to use. It can drop all of your resized, framed thumbnails in a single folder to make it easy to add the images to your latest project. To try Photo Shaman for free or buy it for $45, just go to the Brave Orange site.

Q&A: Where Are My File Extensions?

After installing Jasc Paint Shop Pro, I've found that all of my .tif and .jpg image files are identified in Windows as "Paint Shop Pro images" so I can no longer tell what they are. How do I get them to appear as they really are?

--Mike Wood, Cleveland, Tennessee

By default, Microsoft Windows hides three-letter file extensions. That's supposed simplify your view, but it can be a real pain when you're trying to figure out what kind of files you've got. So I think that the easiest solution to your problem, Mike, is to make Windows display your file extensions.

Here's what to do: With Windows XP, go to the Start, Control Panel, and select Folder Options. (If your Control Panel is set to Category View, you'll find Folder Options in Appearance and Themes.) Click the View tab and find the entry called "Hide extensions for known file types." Uncheck it and click OK.

If you're using Windows 98, open Windows Explorer and select View, Folder Options. Find the entry called "Hide extensions for known file types," uncheck it, then click OK.

Now you should see the extensions so you'll know what file types you're dealing with.

Hot Pics

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: "Sibling Rivalry," by Jordan Swartz, Santa Monica, California

About this week's winning photo, Jordan writes: "This photo made me smile because it reminded me of three siblings, all vying for attention, but one rising above the others to capture the spotlight. While the other two flowers will have their day in the sun some other time, today it was the middle flower's time to shine. I think anyone who has siblings can relate.

"I discovered these flowers in an island in the middle of a street. I took the shot using my Canon Digital Rebel's macro mode. It just goes to show that you never know where you'll encounter your next subject!"

Hot Pic of the Month: Each month we choose one of our weekly winners to be the Hot Pic of the Month. By an odd coincidence, virtually all of April's weekly winners were abstract photos. From those, we chose "Spilled Mercury," by Matt Wilson from Millville, New Jersey for the monthly prize.

Congratulations to Matt and to everyone else who won a Hot Pic of the Week last month. Keep those entries coming!

I want your feedback! Send your comments, questions, and suggestions about Digital Focus to comments@bydavejohnson.com. If you have a question that you'd like to see answered in the weekly Q&A, send it to question@bydavejohnson.com. And be sure to sign up to have the Digital Focus Newsletter e-mailed to you each week.

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