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As you're processing a photo in Camera Raw, the histogram at the top of the basic column automatically updates to give you a live preview of how your adjustments are affecting the tones in the image. Sometime it's hard to judge what's happening with the naked eye by just looking at the preview of an image, so you might think that this photo looks pretty good, but it's actually lacking in contrast, and I can tell that by looking at the Histogram, as we'll see in just a moment. Now this histogram is a lot like the one that we saw earlier in the course in the Levels dialog box, and it's like the histogram that's in the Histogram Panel in Expert edit mode.
It's a chart that represents the tones in the active image. The right side of this black bar represents the brightest possible tones and the left side, the darkest possible tones, with the range of gray tones in between. And the color mound represents the actual tones in this image, with the colors in the mound identifying the contribution of tones in the various color channels. This mound is made up of vertical bars that are set close up next to one another. The height of a bar represents the frequency of the corresponding tone along the bar at the bottom of the histogram. So this tall bar is above a relatively bright part of the histogram, and that means that there's a lot of this particular tone in the image.
As I drag the sliders in the Basic Panel, notice that the shape of the mound changes, so if I lower the Exposure, the mound moves over to the left on the histogram, and if I increase the Exposure it moves over toward the right. And if I drag the Contrast slider to the right, the values spread out on the histogram. Now contrast is a slightly different concept than brightness, so I think it's important to understand the difference. Brightness is just the lightness or darkness, but when we talk about contrast we mean the difference between the dark and light tones. Now many photos will look their best when they have enough contrast, so often your goal is to get a full range of tonal values across the histogram, but that's not always the case.
For example, the parts of an image that are far away usually are lower in contrast than areas that are closer to you, and to force that to be different could make the photo look inaccurate. Notice that there are two triangles at the top left and the top right of the histogram. These are the clipping warnings. They help you to see if, as you're adjusting the photo, you've clipped any highlights or shadows, in other words, whether any highlights or shadows have lost their detail. These triangles are active when there is a small white border around them, so right now the highlight clipping warning is on, and so if I go down to the Highlights slider, and I drag it way over to the right, farther than it should be, and maybe I'll get the Exposure slider and move that to the right too, you start to see areas of red in the image.
They represent the tones that are lacking in detail that are just too bright to have any detail. So if I have the Highlight Clipping warning on and I see this in the image I would probably come down and reduce the Exposure and reduce the Highlights as well until those red bits go away. By the way, when there is clipping in the highlights you'll also see something like this, a spike on the right side of the Histogram. So I will drag those back to get rid of those red bits in the image. Less crucial than the highlight clipping warning but also useful, is the Shadow Clipping warning.
So if I turn that one on and then I drag the Blacks slider way over, you can see some areas in the photo turn blue, and those are the areas that I'm pushing to pure black without detail. And now you see a spike on the left side of the histogram, representing those clipped pixels. So, I might move back on the Blacks a bit to bring more detail into those areas. If those clipping warnings are getting in your way, they're keeping you from seeing the photo, you can turn each one off by clicking on it to remove the white highlight. So, that's a quick look at the Histogram. As you're adjusting a photo in Camera Raw, remember to keep an eye on the histogram to help you understand how your changes are affecting the image, and suggest what other adjustments the image might need.
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