15/08/2015
One of the most valuable tools we have when editing and correcting images in Photoshop is the histogram. In fact, histograms are so valuable, they're not limited to just Photoshop. You'll find histograms in lots of other image editing programs as well, like Adobe Lightroom, Photoshop Elements, and Photoshop's own Camera Raw plugin. Many digital cameras today also include a handy histogram feature. In the world of photography and photo editing, histograms are everywhere!
So, what exactly is a histogram? Quite simply, a histogram is a graph. Now, I know, as soon as I mention the word "graph", some people will want to take a nap, but I promise you, this is easy stuff. It's also very important stuff if you're serious about your images (and if you're reading this, it's a safe bet that you are). A histogram is a graph that shows us the current tonal range of an image so we can evaluate it and, if necessary, correct it. By tonal range, I mean the range of brightness levels in the image. A histogram shows us how much of the image is currently pure black, how much is currently pure white, and how much of it falls somewhere in between.
It's important to note here that even though we're talking about brightness levels, we're not just talking about black and white (grayscale) images. Histograms work equally well, and are equally important, with full color photos. That's because, even though we don't always think of color as being anything more than, well, color, every color in your image has its own brightness level. Yellows, for example, are typically very light; blues are much darker. These differences in brightness values mean that color can have a huge impact on the tonal range of your image.
How does being able to see the tonal range of an image help us? Well, have you ever looked at a photo and thought it seemed a little "flat"? The subject was interesting, the composition was good, but overall, it just wasn't "popping out" at you? Chances are, it's because the image was suffering from low contrast; the highlights (the lightest areas of the image) were most likely too dark and the shadows (the darkest areas) were not as dark as they should have been.
Now, you may have been able to recognize that just by looking at the photo with your own eyes, but when it comes to image editing, the sad truth is, our eyes are not as reliable as you may think. They're great at comparing two images side by side and recognizing which one is better, but when it comes to evaluating a single image, they can be easily fooled into seeing certain areas as either lighter or darker than they really are. Your computer's display (monitor) can also add to the problem if it hasn't been properly calibrated. Setting your screen's brightness too high, for example, can make your images look fantastic on screen but dark and disappointing when you go to print them. A histogram overcomes both of these problems. It doesn't rely on your eyesight or your display settings. Instead, it gets its information directly from the image itself so it's always showing you an accurate, reliable view of the tonal range, making issues such as poor contrast easy to spot and, as we'll see in our Levels and Curves tutorials, easy to fix!
Another common problem we run into with image editing is shadow and highlight clipping, where the shadows are so dark, they're become nothing more than areas of pure black, and the highlights are so light, they're just areas of solid white. You may have heard it referred to as "plugging up" or "crushing" the shadows, or "blowing out" the highlights, but it all means the same thing; we've lost image detail.
Just as with the issue of poor contrast, it's not always easy for us to recognize these clipping problems just by looking at the photo on the screen. Our eyes are simply not sensitive enough, and our computer's display may not be showing us a truly accurate representation of the image. The histogram, however, will always show us if we've gone too far with our editing and need to back things off to bring back detail. Or, if we're restoring an old photograph, the histogram can tell us if the original image itself is missing detail in the highlights or shadows so we know where we're starting from and what we're dealing with. Bottom line, if you're retouching or restoring images and don't know how to read and understand what the histogram is telling you, then you and your photos are at a serious disadvantage.
