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I have more to say about the Black and White dialog box. In fact, I've got a lot more to say, and we'll be talking about it throughout the rest of this course. But before we go back to it, I want to look at another tool in Photoshop, not one that I think you should use, but one that is used in a lot of other image-editing programs and one that if you've been using Photoshop for a long time, you might be used to using, and I want to talk about it because I think if you're an old Photoshop hand who has used this tool before, it's important to explain how it's different from the Black and White dialog. I've got the same exercise file open that we used in the last lesson.
I am going to go to Image > Adjustments > Channel Mixer, and it brings up this thing, which looks kind of like the Black and White dialog box, except I've only got three color channels. To get a grayscale conversion, I have to check this Monochrome box, and that gives me again a default grayscale recipe. Now I've got red, green, and blue. You should be comfortable with what these are now-- the building blocks of any color image. But these are percentages. In the Black and White dialog box, they are simply unit values. My grayscale image is being created by taking 40% of the red channel, mixing it with 40% of the green channel, and then mixing in 20% of the blue channel, for a total of 100%.
As long as this is at 100%, my overall brightness in the image will not change. So let's say I want to darken the sky like I did earlier. I know the sky is blue. So I might say, great, I'll grab the Blue slider and drag it to left. And sure enough, my sky has gotten darker, but so has the rest of my image. And that's because these are not targeting specific colors, these are targeting components. And as we know, every pixel in the image is made by combining these three components. So when I'm dragging the blue slider to the left, I am not saying find blue in the image and darken it.
I am saying darken the blue component. So any color that has blue in it--purple, for example--will get darker because it's got some blue in it. So the sky is getting darker, as is everything else. The other reason that the entire image is darkened is because I've drain luminance from my image. I am no longer at 100%. So to get my image back up to its original brightness, I need to drag these up. So let's say that I wanted to darken the sky. I am going to knock that down to -20. So if I start bumping these up--I am just kind of eyeballing this, trying to get it back up to my original exposure and unfortunately, by doing that I've brighten the sky back up again.
So it looks like there's a red component in the sky, so I should get my brightness back with green--and that's still getting a little darker. Are you beginning to see why I don't push this tool? The reason I'm explaining it though again, is for those of you who've used it before. This is what is different about the Black and White dialog box is it's not a component mix. It's actually identifying specific colors in your image and toning them. If you do find yourself editing color in another image editor that only offers a channel mixer, then these are the kinds of problems you are going to be facing.
Yes, you can try to target a specific component to lighten or darken it, but if you do, you're going to be affecting the rest of your image. This Constant slider down here let's you apply an overall brightening or darkening to your image, but it's a pretty blunt instrument. As you can see, as I brighten, my darks get lighter also, so this is not a real useful tool. Nevertheless, I'm hoping that by looking at this, you now understand the black and white is not affecting component colors. It's actually affecting discrete colors in your image.
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