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You may have heard the term histogram and be wondering exactly what it means. In Photoshop, the Histogram is a panel that contains a bar chart that represents the distribution of tones in an image. If you know how to read a histogram, it can be useful when you're correcting colors and tones in a photograph. In this movie, I'm going to explain how the histogram works in Photoshop and show you some examples of photographs that have different kinds of histograms. If your Histogram panel isn't open, go to the Window menu at the top of the screen and choose Histogram, or you can change to the Color and Tone workspace from the Workspace menu, as I've done here.
I usually go to the panel menu on the Histogram and change it from its default Compact View to Expanded View. Right now the histogram is showing us information about all the color channels in the special Colors view. To make it easier to read, I'm going to change the Channel menu there to look at an RGB Composite View. So what is the histogram? As I mentioned it's a bar chart that represents the distribution of tones in the open image. The left side of the chart represents the darkest tones in an image. The right side of the chart represents the brightest tones in an image and the area between represents all the possible gray tones in an image.
This black mound here represents the actual tonal values in the open photograph. If you could pull this mound apart, you would see it's made up of individual vertical bars. Each of those bars is above a particular point on the graph. Where there is a tall bar, that means there is a relatively large amount of that particular tone in the image, and where there are no bars, that means there's none of that particular tone. Every photograph will have a different histogram. You don't always want a histogram that looks like this, but many photographs do look best if they have a wide range of tones.
Those photographs like this one will have a histogram that runs across the entire Histogram panel. But let's take a look at some other kinds of photographs and their histograms. I'm going to click on the second tab that I have open here. The over.psd image. You can see by looking at it that this is a very bright image. If you look at the histogram for this photograph, you'll see that all of its tones are indeed over at the white side of the bar graph. There are very few grays and no blacks at all. Let's look at the clipped.psd image.
The foreground of this photograph looks fine, but as you can see, many of the clouds are pure white, lacking in detail. That's represented on this histogram by this spike on the far right. If you have a spike in an image, that's usually not a good thing because it means you have lost detail, either in the highlights if the spike is there or in the shadows if the spike is over here. You'll notice something else in this histogram. There's a yellow triangle here. That means that the histogram has changed and needs to be updated. You see this in Histogram panel when you make an adjustment to an image.
To update the histogram you can click on this double curved arrow and the yellow warning symbol will go away. Let's take a look at a really dark image. Its histogram is located primarily in the dark area, but there is a lot of middle gray too. If you'll notice in the Layers panel, there are two layers in this image. The top layer is composed only of this dark bank building. I can set the histogram to show me just the tones in a single selected layer by going to its Source menu and choosing Selected Layer instead of Entire Image.
Watch how the histogram changes when I do that. Now as expected, we see that all of the pixels are over on the left side representing just the dark pixels in the bank layer. I'd like to show you one more image and that is this one, flat.psd. I find the histogram particularly useful on an image like this, because when I look at it, I can see that it doesn't look very good. But I'm not necessarily sure how to fix it. It helps me to see in the histogram that all of the pixels in this image are concentrated in the gray tones in the middle. That means it doesn't have much contrast.
In other words, it doesn't have white whites and black blacks. So, if I wanted to correct this image I would probably do a Levels or a Curves adjustment, trying to expand this tonal range and get some whites and blacks and different shades of gray into the photograph. The histogram really is a useful tool that can help you to analyze an image when you're beginning to edit it and to understand what your edits are doing as you make them. Now that you've seen a few examples of different kinds of photographs and their histograms, I hope you'll be better able to read the histograms on your own images.
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