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In this movie, I'll introduce you to another way to gauge the luminance levels inside your image and it's called a histogram. Now at first it might seem quite technical, but once you come to terms with the histogram, luminance levels make that much more sense. Now to see the histogram, you go onto the Window menu and you choose the Histogram command which brings up the Histogram panel. Now what this is, it's a bar graph of the various luminance levels inside the image. You may see it in color. You may see it in white. To make things a little less confusing, what I'd like you to do is click on the flyout menu icon and choose Expanded View in order to increase the size of the graph, and then go ahead and switch the Channel from Colors to Luminosity, so we can see the core luminance levels inside the image.
Now at first glance you may look at this thing and think how in the world is this going to benefit me? Well I've created a diagram of a histogram for you and I'll walk you through it so that it makes more sense. I'm going to hide the Histogram panel for now. We'll come back to it in the next movie when I show you a practical application of the function. And I'm going to switch to this image called histogram.psd again, found inside the 07_luminance folder, and I'm going to press Shift+F. By the way, you can back up through the Full Screen modes by pressing the Shift Key along with F and that'll take me directly to full screen as you see here.
And this is a big diagram of a histogram. Here's how it works. This is a bar graph of the luminance levels inside of your image, starting with black over here in the far left-hand side, and going all the way over to white on the far right-hand side. And so it's ultimately a kind of popularity contest; the taller the line the more of that specific luminance level you have. To get even more technical, your standard digital image is an 8-bit per channel image.
What that means is you have up to 256 different luminance levels, including black and white, and all the other luminance levels in between, per channel. Some images have more than that but that's the standard. And so if you were to take a careful look at the histogram and count up all of these bars here, you'd find that there are a total of 256 bars in all. Each one of these luminance levels has a specific numerical value associated with it.
Black is 0 and White is 255. Now that may not make sense. After all, I just told you there are 256 luminance levels in all. How is it that white at 255, plus black at 0, adds up to 256? Well it's because black is yet another luminance level that's just sitting there at 0. So you've got 1 through 255, plus black at zero. That gives you 256 in all.
Now when you're reading the histogram, this area over here is going to be the shadows, as I've labeled, so the left- hand side, that's where the shadows are at; the highlights are going to appear over here on the right-hand side; and then the midtones are going to appear in the middle of the graph. And again, these are just rough general definitions of those regions of luminance level. What you want to see is that the graph pretty much starts right at the beginning here and slopes up, and then we have a healthy amount of shadow detail.
You also want to see over here in the right-hand side that the graph amps up at white and that we have a healthy number of highlights going on, and then finally, you want to see a lot of bouncing inside the midtones. What you don't want is to see a big spike right there at black or a big spike right there at white with relatively little action going on in the middle of the graph, because what that tells you is that you have a lot of clipped shadows and you have a lot of clipped highlights, and when you run into an image like that you can make it look a little better, but you're never going to make it look great.
It's pretty much a failed image from the get-go, and you certainly don't want to take an image that has a histogram like this one, a nice healthy histogram that is, and turn it into one where the middle of the graph is very low and then you have spikes at either side. And of course as with any bar graphs, small bars mean you have few luminance levels at that location and big bars mean you have lots of luminance levels. Now by lots I don't mean any specific value, because Photoshop goes ahead and scales the histogram according to how many luminance levels it finds throughout the entire image.
And so that's how the histogram works. You'll find it inside the Histogram panel. You find it elsewhere inside the software, as well. And once you get a sense for how it works, it's an extremely helpful tool. And I'll show you how to use the histogram to gauge the quality of your correction in the next movie.
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