Whenever I want to make basic contrast adjustments, I usually do not use the contrast and brightness option. What I choose is levels - under the image menu>adjustments>levels. And this will bring up our levels dialog box. Notice what we have here is what is called a histogram, and this histogram describes the brightness values in my image, and the amount of varying brightness pixels. At the far left of my histogram graph here is a black slider, then we have the gamma or middle gray slider. And then the white slider. So you can see that the dark values for my image are over here, to the middle values, to the highlight values. So I can determine just by looking at my levels dialogue box, that I really don't have any bright white values or very many dark values. So just by looking at this particular graph, I can see that I have a fairly low contrast image. At the very bottom of the levels dialogue box, I also have some sliders that allow me to control the output levels. I want to talk about how we can adjust the contrast using our levels sliders. I can drag the black point up to where the dark pixels really begin, right here. And notice what is happening, of course as I drag this over, it's making my image appear a bit darker. Also make sure that you have preview enabled to see this happen dynamically in your picture in the background. So I'm going to do the same thing for my brightness or white point slider, by clicking and dragging that slider down to where the bright pixels really begin. And we could also adjust the mid-point values of my image by clicking and dragging on the mid-point slider. So if we drag it to the left, of course it's making my image a bit lighter, because it's saying that the middle values are now in a darker area. So that means that anything to the right of this will be brighter than the middle value. And conversely, if I drag it a bit to the right, it will make my overall image look at bit darker. If you wish, we could also select any of these numerical values right here and type in our own value - just type in 20. And notice, drag this out of the way, how my image looks. It has increased the contrast for it. I'll go ahead and click ok. And apply those changes. And I'll undo them so you can see the effect. So it's definitely helped my contrast a bit. I'm going to go back to image menu>adjustments>levels and talk about another way that we can use the levels dialogue box. If you wanted to make your image very contrasty, then of course we would squeeze these sliders closer together. And indeed it really is popping up that contrast. Remember at any time if you want to reset these values to their default settings, all you need to do is hold down the options key on the Mac or Alt key on the PC and the cancel button will return to reset allowing me to zero out any changes, so that I can start over. Another way you might use levels is by selecting the black point or white point by clicking right in your image. So I'm going to select the white point by clicking on my white eyedropper tool, and moving into my image and choosing what I think would make a nice white point, like the side of this building here. And notice when I click it, the actual color changes, because that side of that building had a bit of a color cast to it, it was slightly blue, so in order to neutralize that blue, it warmed up as well as stretching out my histogram, so that that would now be the brightest value in my image. And to the same extent, I could place my black point in this image by choosing the black eyedropper tool. I'm going to drag the levels dialogue box out of the way and look for a very dark area, maybe the shadow here and click on that. If you wanted to reduce the contrast overall, we could do that by effecting the output levels. So normally the output levels are taking advantage of the entire range of brightness values. But you can see that if I drag my black output level slider up, this means that no pixel can be darker than this value. And to the same extent, I could drag the white point output value down, and notice what it's doing - it's really reducing the contrast, because, of course at this point right here, this means that no pixel can be brighter than this value, so we are reducing the contrast. I'm going to zero out these values again and just stick with these changes that I've made which have enhanced the contrast tremendously.
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