From the course: Photoshop One-on-One: Fundamentals

How luminance works - Photoshop Tutorial

From the course: Photoshop One-on-One: Fundamentals

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How luminance works

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- All right, I'm going to start things off by introducing you to some terms and ideas that will help you understand how luminance works inside of a digital photograph. And these terms and ideas will not only help you better understand this course but they will help you better understand how to adjust luminance inside your own photographs. And so here we are looking at the most drab washed out photograph conceivable, hopefully worse than anything you'll run into. And yet we can fix this image just by choosing a command. Now, even though it is so drab, it's quite obviously a full color image, but not to Photoshop. And that's because 99 percent of the time Photoshop is seeing gray scale only luminance data. Starting with the darkest color is black and the whitest color is white and at least 254 shades of gray in between. Now these luminous ranges have common names. The darkest stuff is known as the shadows. The brightest pixels are known as the highlights, and the stuff in between is known as the mid-tones. Now there's no specific border between shadows and mid-tones or mid-tones and highlights. It's a continuous spectrum in between. Now because this photograph is so very drab. I'm identifying what paths for shadows in this image with black, and I'm identifying what paths for highlights in white, and then I'm setting the mid-tones to purple as we're seeing right here. And yet just by identifying the shadows highlights and mid-tones, we end up with a more interesting image. Now as with any everyday average digital photograph, we have three channels of primary color information. Starting with red, second, we have green and then we have blue. And these are the primary colors of light. And so as they mixed together, as we add more primary color, we get brighter pixels. Where the highlights in the red and green channels intersect, we end up with yellow. Where the highlights in the green and blue channels intersect, we end up with cyan. And then we're the highlights in the blue and red channels intersect, we end up with magenta. When we have highlights in all three channels, we end up with white, and where we have shadows in all three channels, we end up with black. All right, so let's take a look at what's going on with this particular photograph right here. Now, I've gone ahead and identified the brightest spot in this image inside this little black circle, and the darkest spot inside this white circle. So we can evaluate exactly what's going on. Now you want to make sure that your color panel is visible on screen, and I'm also going to go up to the window menu and choose channels, in order to switch to that panel as well. And notice, in addition to this item called RGB, which is the full color RGB composite image, we also have the independent color channels, red, green, and blue. And I want you to notice this granite egg right here in the full color image, it's a little bit brown which is a kind of yellowish orange. And so it's going to be brightest in the red channel, and then it's going to start dimming down in the other channels until in the blue channel, we're seeing this very interesting reflection here. Now because the sky is blue, it's showing a brightest in the blue channel and it's darkening up inside the red channel. All right, I'm going to go ahead and grab the eyedropper, which you can get by pressing the I key incidentally. Now this tool has a hotspot right at his bottom left tip, but if you're interested in measuring things a little more exactly with this tool, which is a color measuring device, then you want to tap the Caps Lock key. So you get this target cursor right here. All right, now I want you to see that little dot here inside the color field. Notice that currently the foreground is active and the dot is all the way in the bottom left corner, meaning that we're seeing black. But if I go ahead and click here in order to lift that color, notice that that circle bounces quite a bit higher because we have a brighter color. All right, I'm going to click on a fly out menu icon in the top right corner of the panel and switch to HSB sliders. HSB stands for Hue Saturation and Brightness, by the way. Because we're working in a single channel, all we're going to see is brightness, which is the core luminance data. And so black has a brightness value of zero percent, white has a brightness value of a 100 percent. This darkest color inside this image has a brightness of 38 percent. And so, if I were to bring back my black to white gradient that's going to be somewhere around here, actually even brighter still. So right here at this point, meaning that everything to the left of my cursor is missing from this particular image. All right, now I'll go ahead and click in the brightest point right here, which gives me a brightness value of 84 percent. So remember a 100 percent is white, which means that everything from the right of my cursor is missing inside this channel. And so, I've gone ahead and identified those two points just so that you can see, this is the limited range of luminance that we have inside this specific channel where this very washed out images concern. All right, so I'm going to go ahead and switch back to the RGB composite, and I'll switch back to the layers panel as well. If you're working along with me, you may want to go ahead and turn off the Caps Lock key because otherwise it might be confusing when you start using other tools. And then I'm going to correct this photograph using what has to be the most straightforward color correction tool in Photoshop. You go up to the image menu and you choose auto contrast. And what it does is it makes the brightest color white, and the darkest color black on a composite basis. And I'll explain what's going on there in a future movie but for now, I'm just going to choose the command and just like that we have corrected the photograph. And now we have access to the entire range of luminance data on a channel by channel basis. And so again, just so you have a chance to see the difference. This is the original washed out version of the photograph and this is the version we get by choosing a single command auto contrast available from the image menu. And that is how luminance works on a channel by channel basis. Not only in the narrow world of Photoshop but across the wide world of digital imaging.

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