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It's one thing to gauge your Brightness and Contrast adjustments onscreen and just accept that your screen is accurate and that your eyes are doing fine today, but it would certainly be useful to have another tool at your disposal, that you could use as a check to make sure you are doing a good job. That tool is called a Histogram. Now, it's a little bit techy, but even if you can just mildly wrap your brain around it, you are going to have a lot more success inside Photoshop. It's this guy right there. You can click on this little Histogram icon to bring up the Histogram panel, or you can go to the Window menu and choose Histogram.
You may look at this little guy and just think, what in the heck is this, and how in the world is it the least bit applicable to what I am doing? Which is why, before I show you how the panel works, I want to introduce you to the Histogram itself. So I have this image right here, this diagram, called Histogram.psd. It's found inside the 07_basic_correct folder. I am going to press Shift+F in order to fill the screen with this image. This is the large view of a Histogram like the one we just saw a moment ago in the Histogram panel. What we are seeing is a bar graph of all the Luminance Levels inside of an image.
Now, by Luminance Levels, I mean Brightness levels, starting at Black, over here on the left, and progressing all the way up to White, over here in the right. Now, not to geek out on you too much here, but in the standard 8-bit per channel digital photograph, which is your standard JPEG image by the way, there are a total of 256 different luminance levels in each of your color channels: Red, Green, and Blue. That's why if you were to sit down and count the number of lines inside of the Histogram, you would find that there is exactly 256.
Again, Black being the darkest, White being the lightest. These guys also have numerical values associated with them. Black is 0 and White is 255. Now, how in the world does that add up to 256? In other words, 255 plus 0 is still 255. Well, the idea is there is 1 through 255 for everything but Black, and then add Black and you get 256 different Luminance Levels. Now, the height of each one of these vertical bars indicates how many pixels are in that particular Luminance Level, anywhere from None.
So notice in our case, we don't have any really super dark colors over here, because we have no vertical bars, not until this point right there. So we are just starting to get some Shadow regions, all the way up to, notice this block of Highlights right here. We have got Lots, because it's topping out at the top of the graph. Now, Lots is a pretty vague concept, but the graph scales depending on how many pixels are actually inside of your image. So None down here. Lots up here.
Now, I have also divided up the various Luminance Levels into their three basic regions, which are known as Shadows for the darkest stuff and Highlights for the lightest stuff, and then Midtones for everything in between. If you saw the way I just dragged my Marquee. You see that there is a lot of overlap between Midtones and Highlights as well as Midtones and Shadows. There is no exact Luminance Level at which we stop having Shadows and we start having Midtones, they are just general regions of luminance inside of the image.
So what we are seeing where this Histogram is concerned is that we have very little in the way of Shadows. We have got quite a lot of Highlights, and then we have a reasonable number of Midtones as well. The more balance you can achieve between your Shadows, your Midtones, and your Highlights, the better your image is going to look typically. That's just a rule of thumb. Of course you violate that rule all the time. But better balance means better looking images. We can also see that we haven't topped out in the Whites, because there is nothing going on right there.
We haven't bottomed out in the Shadows. So we haven't clipped any colors. If we were clipping, so that big regions were going to Black or White, which is the ultimate Shadow and Highlight color, then we would see a long line over here on the left-hand side for Black or we would see a long straight vertical bar that is on the right-hand side for White. So this is a good looking Histogram, all things considered. I just want you to get a sense of what's going on there. It will make more sense over time. In the next exercise, I will show you how to apply this knowledge to using the Histogram panel inside Photoshop.
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