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Before you correct the tonality of an image it's important to diagnose the problem that you want to correct. It takes practice to train your eye to read the tonal values in a photograph and even if you are skilled in the art of reading photographs, there will be times when you'll find it hard to determine what's wrong with an image by relying on visual cues alone. That's when the Histogram panel can really come in handy. The Histogram panel offers an invaluable way to evaluate photographs to diagnose problems that you want to correct and to monitor the effects of adjustments as you make them.
Let's start by opening the Histogram panel. You can do that by going up to the Window menu at the top of the screen and choosing Histogram or as I did, you can change you workspace to one that includes the Histogram panel like the Color and Tone workspace. By default the Histogram panel looks like this when you open it. I prefer to see it larger and so I like to go the panel menu on the right side of the Histogram panel group and choose Expanded View. In Expanded View I have a menu up here that I can use to change the way the channels in the image are represented.
By default the Histogram panel represents each one of the Color Channels in the image. I think it's easier to read the Histogram, if you go into the channel menu and you choose RGB instead which presents a composite view of the Red Green and Blue channels in an open image. The Histogram in this panel is basically a diagram or a bar chart of the tonal values in the open image. The left side of the Histogram represents the darkest tones in the image and the right side of the Histogram represents the brightest tones in the image.
The mound in between represents the Gray values in between the brights and the darks. If you could pull this mound apart, you'd see that it's made up of individual vertical bars. And each vertical bar represents a particular shade of Gray in the image. The height of a bar varies with the relative frequency of its tone in the open image. A tall bar like these in the middle of this histogram mean that there is lots of that particular tone in the open image. If there are no bars, in an area of the image or very few like those over here on the right, it means that there is not much of the corresponding tone in the image, so in this particular image, there isn't much white.
Many photographs look best if they have a wide range of tones and good contrast between the blacks and the whites, like this photograph does. So it's often your goal to expand the tonal range of a photograph as you are adjusting it. Now let me show you an image that doesn't have such a good tonal range. I'm going to click on lifeguard.psd, which is open in this tab. This image looks pretty flat when you view it and if you go over to the Histogram panel you can see why. There are lots of Gray pixels in the image and some dark pixels as well but there are no bright pixels at all. And that means there is very little contrast in the image because contrast refers to the difference between the bright brights in an image and the dark darks.
This could probably be fixed with an adjustment like Curves or Levels but before you apply an adjustment like that, you need to analyze what the problem is and the Histogram panel can help you with that. Now let's look at another image restaurant.psd which is also open here. In the histogram, it looks like there is a pretty good tonal range in this image from darks all the way over to brights. But there's one thing about this histogram to be wary of and that's the spike over here on the far right. A spike like this means that that area of the photo is clipped.
In other words, in this case, that the detail in these highlight areas is missing from the photograph. If you happen to be adjusting a photograph and you notice a spike either on the right side representing clipped highlights or on the left side representing clipped shadow areas, you might want to pull back a bit and your adjustment to avoid losing detail in those areas. Let's look at one more image, fence.psd. I'm showing you this image because by default the histogram represents the tones in an entire image. But if you want to, you can change the histogram to represent the tones in just a single layer.
I'm going to go down to the Layers panel here which has collapsed to make room for the histogram panel and I'll Double-click the Layers tab to expand the Layers panel again. Here you can see that there are two layers in this file. The foreground layer, which contains all this foreground image along with some transparent pixels, and the sky layer, which contains just the dark sky and some transparent pixels. Now if I select the sky layer in the Layers panel and I then go up to the Histogram panel, look how the histogram changes. If I change the Source of the Histogram from the Entire Image where there's a wide range of tones to the selected Sky layer. The histogram is now confirming that in the sky layer most of the pixels are gray or dark and there are no light areas at all. You can use this information if you are adjusting just the sky.
So as you can see the histogram is a really useful tool which can help you to evaluate an image before you adjust it and to understand the effects of your adjustments on an image as you make them. If you do have room on your screen, I'll suggest you always try to keep the valuable Histogram panel open whenever you are adjusting tone in an image.
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