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In this movie, I'll introduce you to a few terms and ideas so that you understand how luminance works inside of a digital image. And these ideas will not only help you understand how to correct luminance throughout this chapter, but also when you're working on your own images in the future. I'm working inside a file called luminance demo.psd, it's found inside the 07_luminance folder. Now even though we think of an image as being full-color, it's really a combination of grayscale images working together.
And I'll show you what that means in just a moment, but in the meantime know that every pixel has a luminance level, from black at the darkest to white at the brightest, and these ranges of luminance have general names. The darkest luminance levels are known as the shadows, the brightest luminance levels are known as the highlights and then the luminance levels in between are known as midtones. Now there's no specific place at which highlights end and midtones begin, or midtones end and shadows begin.
These are just general ranges of luminance. Now as I was saying, what we see as a full-color image is actually multiple grayscale images working in concert with each other. These grayscale images are known as channels. This image, like all digital photographs contains, three channels, we have a red channel, we have a green channel and we have a blue channel. Where the bright colors in the red and green channels intersect you get yellow. Where the bright colors in the green and blue channels intersect, you get cyan, and where the highlights in the red and blue channels intersect, you get magenta.
Just to give you a sense of how these channels mix to form the full-color image. If you have highlights in all three channels you get white, if you have shadow in all three channels you get black. Now let me show you what the channels look like where this specific image is concerned. I'm going to go up to the Window menu and choose the Channels command in order to bring up the Channels panel which by default lives next door to the Layers panel. And notice that we're seeing what's known as the RGB Composite; that is red, green and blue working together and that the red, green and blue channels are all selected because they're all turned on.
However I can click on any one of these channels to view it independently. So for example, I'll click on the red channel and as you can see, it is a grayscale image. This is what Photoshop sees as it evaluates a full-color image, because Photoshop sees and addresses the image one channel at a time. And as you can see, where this image is concerned, we have tons of highlights inside the red channel. We have a few midtones here and there, but we really don't have anything along the line of shadows.
And just for reference, I'm going to turn that gradient back on, and you can see that the darkest luminance level inside this channel is somewhere around here inside the gradient. So it's by no means black, which is why we have such a washed out image in the first place. Now let's take a look at the green channel, and you can see that things darken up but still not enough, and then here's the blue channel, darker still, but also very bright. All right now, I'll go ahead and switch back to the RGB image and I'll go up to the Image menu and I'll choose a command called Auto Contrast.
And this is one method for correcting the luminance levels inside of an image. And notice that Photoshop darkens up the image considerably and again it does so on a channel-by-channel basis. So every one of these channels is darker. And in fact what Photoshop has done is it's taken the darkest pixels inside the image, which were quite light, and turn them black and then stretch the other luminance levels across the gradient spectrum. And so if I take a look at the red channel now, you can see we've got some very dark shadows inside the pupil, in the eyelashes, and around the iris, and so forth.
The same goes for the green channel, which is darker still, have some very rich shadows going on, and then in the blue channel the same is true except we have more shadow detail than ever. And that friends is how luminance works here inside Photoshop.
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