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The Curves adjustment in Adobe Photoshop has a reputation for being challenging for some photographers. In this workshop, Photoshop expert Tim Grey takes you step by step through every aspect of the Curves adjustment, helping you truly understand the concepts behind it so that you can quickly and easily maximize tonal range, optimize contrast, and enhance your photos' color balance. Note: This course was recorded in Photoshop CS5, but was created with users of both Photoshop CS5 and Photoshop CS4 in mind.
Most images benefit from having a maximized tonal range, meaning the darkest pixels in the image are black or nearly black and the brightest pixels are white or nearly white. With a proper adjustment of the black and white points, you can accomplish excellent contrast in an image without losing any highlight or shadow detail, as you'll see in this lesson. The black and white points are adjusted by changing the position of the endpoints on the curve. The left endpoint adjusts the black point in the image, and the right endpoint adjusts the white value in the image.
What you're really doing is remapping the before and after values. In other words, if I move the black point over to the right, what I'm saying is that any pixel that has a luminance value, a brightness at or below this value will be shifted to pure black. Of course, if I move my black point over too far, I'll lose considerable detail within the image. You can see my shadow areas in the image have gotten completely blocked up. That's because any pixel that have this particular value or lower has been shifted to pure black. All the areas of the dark shadows that had subtle variations in tonal value have now been completely shifted to black.
All the detail there has been lost. The key then is to apply an intelligent adjustment when working with the end points to establish black and white values. Now, you can use the histogram for this purpose. You can see here, I have the histogram display turned on in curves. And so, I can see where the brightest pixels end and where the darkest pixels end. So, in theory, I could simply position my endpoints so that they align with the edge of that histogram. I don't recommend this approach, because it means you're not making use of all of the available information within curves.
Sure, we could use this histogram and it's reasonably accurate and helpful, but it doesn't give us quite as much information as I'd like to see. I also don't actually manipulate the endpoints themselves. Rather, I use the sliders found below the Curves adjustment area, which will accomplish the same thing, but give me access to one additional feature. And that is the Clipping Preview. I'm going to go ahead and reset my curve here just to get back to my original starting value, and will adjust the black and white points using the Clipping Preview Display.
To do so, I'll simply hold the Alt key on Windows or the Option key on Macintosh, while adjusting the black or white point. However, you must use the sliders below the Curves adjustment rather than the endpoints in order to enable the clipping preview display. So, I'll hold the Alt or Option key and then click and drag, moving my white point inward. As you can see, even though the histogram doesn't show that there are any pixels in this very bright area, I already have some clipping showing up within the image.
That area of red indicates that one channel, the red channel, is losing detail based on this adjustment. Of course, I can't necessarily make an evaluation of whether that's a problem by looking strictly at the clipping preview. The Clipping Preview Display shows me all black except where clipping occurs when I'm adjusting the white point. But I can release the Alt or Option key and then continue moving the slider in order to see where in the image that actual clipping is occurring. In this case, I obviously can see that it's just a headlight and I'm not terribly concerned about losing detail in the headlight of the car. So I might press the Alt or Option key again, and then slide a little bit further, and see what else I might clip if I take my adjustment a little bit further.
Of course, in this case, you can see the sky is starting to get clipped. I'm losing detail in areas of the image that are blue and cyan. I don't want to lose any detail in the sky, and so, I'll bring my slider back over to the right, to the point where the pixels in the sky disappear indicating no clipping is occurring there. The Clipping Preview Display allows you to make a more informed decision about the adjustment you'll apply to your black and white points. The same process works equally for the black point. I'll hold the Alt or Option key, and then move the slider for the black point. You can see right away that there is some clipping occurring in the car and if I take this adjustment further, I'll start to see clipping in more and more areas. In the case of either the black or white point, the color values indicate which channel or channels are losing information.
Black, when adjusting the black point, indicates that those pixels have indeed gone to pure black. And with the white point adjustment, white indicates that those pixels have gone to pure white. In most cases, I try to minimize clipping. I want to maximize the tonal range within the image, and optimize contrast, but I don't want to sacrifice information in the process. In some cases such as with a silhouette, you might want to lose a lot of information. You're giving up information for the benefit of the appearance of the image.
The point is that we're able to make an informed decision through the use of the clipping preview. With the black and white points established for the photo, you've achieved an optimal tonal range and you're ready to move on to finetuning the curves adjustment to produce the best overall result.
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