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The histogram

From: Photoshop CS6 Quick Start for Photographers

Video: The histogram

A photographic exposure, obviously, relates to how much light is being capture by the camera. And with a digital camera, or with a digital file, in any event, we're able to actually view the distribution of tonal and color information, within an image. And that, can be helpful, when we're evaluating an image, and when we're applying adjustments to a photo. Let's take a look at the histogram which shows us the distribution of tonal values in an image. I'll start off by choosing Window and then Histogram from the menu, that will bring up the Histogram panel.

The histogram

A photographic exposure, obviously, relates to how much light is being capture by the camera. And with a digital camera, or with a digital file, in any event, we're able to actually view the distribution of tonal and color information, within an image. And that, can be helpful, when we're evaluating an image, and when we're applying adjustments to a photo. Let's take a look at the histogram which shows us the distribution of tonal values in an image. I'll start off by choosing Window and then Histogram from the menu, that will bring up the Histogram panel.

I'll go ahead and drag it out so that it's floating. And I'll close the navigator panel that came along with it. Now at first glance, the histogram might look like in this case, just a jumble of colors. Let's simplify things just a little bit by expanding this view to include all of the channels so that we can see the individual details on each of the channels in the image. We'll take a look, for example, at the red channel. With the red channel, we can see that there are a lot of midtone values. And a small number of highlight values.

And a small number of dark values of shadows or blacks. The histogram goes from left to right, black to white. So, the far left of the histogram represents black. And the far right represents white. In between, all of the Tonal Values are represented. In this case, I'm looking at the Red Channel so it reflects the Tonal Values found on the Red Channel. In other words, how much red light is included in the photo. The height of the Histogram at any point indicates the relative number of pixels at that Tonal Value. So, here you can see on the Red Channel there are no black pixels and there are no white pixels. There are a few pixels,a relatively small number of pixels. That are reasonably dark, not quite black but still quite dark. And on the white side of things you'll see a similar situation. We have a small number of pixels that are quite bright, but non of them are white. Now, once again, this is reflecting red light at the moment. But we're just talking about the concept of histograms in general. Here we see most of the information is at the center, mostly midtones, and that's reflected throughout all of the channels here.

The red channel has mostly midtones, the green channel has mostly midtones and the blue channel is midtones but they tend to be a little bit darker. If you take all of these combined you'll get the color histogram shown above and at the moment I'm looking at the colors view. And if you look closely you might notice that, that is literally just the individual color channels stacked one on top of the other, and shown in their respective colors. Where they overlap of course the colors blend, and where they're shown in grey all 3 channels are represented there.

So, we have a lot of red pixels and those are the brightest values in the image. We have a lot of green pixels in the midtone range, and the blues are mostly in the darker range, which actually accounts for much of the yellow in the image, with yellow being the opposite of blue of course. I can also change the histogram display from colors to RGB, which I never recommend using. We can do just the red channel, just the green channel, just the blue channel. Or the overall luminosity. Now the luminosity is literally reflecting just the overall brightness values in the image. In this case it looks quite similar to the green channel, but it's not exactly the same as the green channel.

It simply reflects the overall luminance values in the image. In other words, take away the color, and then evaluate the brightness values in a photo. Of course this raises the issue of, how much do we really need to pay attention to the histogram? I mean, I can look at the picture and get a sense of how many bright pixels and how many dark pixels, and how many in-between pixels there are, so what is the real value of the histogram? Well for one thing, it gives us a sense of the overall exposure. With digital cameras, the best exposure in terms of maximizing detail and minimizing noise will be as bright as possible without losing any highlight detail.

So you can see, this image could have been exposed a little bit brighter. The histogram display can also give us a sense of the adjustments we might like to apply to an image. We can see for example, that there's a lot of midtone values, but they tend to be toward the dark side of things, and so maybe I can brighten the image up just a little bit, or, enhance contrast a little bit. A Histogram that is spread out very wide indicates an image that has strong contrast. One that's a little bit more compressed suggests low contrast. So, this image, all things considered, represents slightly low contrast.

That doesn't mean I need to increase contrast. Again, it's just about reading information about the photo and getting a better sense of that information. As we're applying adjustments, from time to time, the histogram display will be especially important. I don't tend to view the histogram for images all that often, unless I have an concern about exposure. For example, thinking I might have blown out some of the highlight detail. However, I do use the histogram when applying adjustments, especially for the levels and curves adjustments that you'll likely become familiar with as you continue using photo shop to optimize your images. The key is to be aware of the various information that a histogram can present about an image.

It gives you some sense of exposure, of overall luminance values, of contrast, and other factors that you may want to take into account as you work with an image. Of course, when I'm viewing a Histogram on the Histogram panel, I usually want to minimize the amount of space being consumed, and so generally speaking, I'll work with the colors display and I'll switch to the compact view so that I can see just that histogram that represents all 3 of the individual color channels. Note, by the way, that the histogram is generally based on cached data, so it doesn't always reflect every change in your image.

That's indicated by the exclamation point on the triangle here. And to update the histogram to reflect the latest updates to the image, you can simply click that button, in order to refresh the histogram, an remove the alert symbol. So as you can see, the histogram is a relatively simple display. Sometimes it takes a little bit of translation to understand exactly what's going on an what it means about your photo. But looking at the histogram from time to time, especially when you have concerns about exposure, can be very, very helpful as you think about your images and consider some optimization approaches you might want to take.

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Photoshop CS6 Quick Start for Photographers

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