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Photoshop CS4's adjustment features offer unparalleled opportunities to correct and manipulate images. In Photoshop CS4: Image Adjustments in Depth, Jan Kabili explains how to use all the major Photoshop adjustment features. She shares the best techniques for adjusting image quality, and shows how to use the new Adjustments panel to streamline a photo correction workflow. Jan also demonstrates multiple ways to eliminate color casts, and explains how to use the new On-Image Curves control to adjust brightness and color. This course offers a detailed look at the techniques photographers and designers use to master image adjustments in Photoshop. Exercise files accompany the course.
One of the adjustments in Photoshop that you may not be familiar with is the Threshold adjustment. This adjustment takes all the pixels in an image and pushes them to either pure white or pure black. One use for this adjustment is to add a graphic look to a photograph like this one. I'll show you how in this lesson. I'm going to add a Threshold adjustment not as a direct adjustment, but as an adjustment layer. So I'll go to the Adjustment panel, and I'm going to click here on the Threshold icon. That adds this Threshold adjustment layer. And it really changes the look of the image. All of the pixels in that photograph now appear either appear pure black or pure white.
To understand why, take a look at the Adjustments panel. Here there is a histogram of the tonal values in the underlying photograph. And those range from level 255 pure white on the right, to level 0, or pure black on the left. There is a slider here that's smack dab in the middle at level 128. And this adjustment is taking all of the pixels in the image that are lighter than value 128. All of those over here in the histogram, and pushing those to pure white, and then it's taking all of the pixels darker than that middle 128, all of them over here, and pushing those to pure black.
The histogram tells us that there were more dark pixels in the image to start with and so there is now more black. I can change that balance by moving the slider in the Threshold Adjustment panel. So if I take that slider and I drag to the left, I'm reducing the number of black pixels, and increasing the number of white pixels. Because there are now more white pixels on the right side of this slider. Now I'm going to use a layer blend mode. To blend this Threshold view with the photograph on the layer below, I'll make sure the Threshold adjustment layer is still selected in the Layers panel, and that I'm going to go to the Layer Blend Mode menu and click there, and I'm going to go down and choose the Overlay blend mode. And I think that gives a really nice graphic effect, with the black pixels on the Threshold adjustment layer, outlining some of the tones and colors showing through from the photograph on the Background layer below.
So that's one interesting way to use a Threshold adjustment layer.
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