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The curves adjustment includes three eyedroppers that allow you to adjust the black point, white point, and neutral gray value for a photo. For certain images, these adjustments can be very helpful for insuring accurate color and optimal tonal range within the image. Let's take a look at how they work. The basic operation is actually quite simple. We'll start with a black eyedropper. Simply click on the black eyedropper tool in order to make it active and then click in an area of the image that should be perfectly black. The individual color channels will be adjusted so that the area you clicked on is perfectly neutral black.
Not nearly black or black with a little bit of a color cast, but pure black. Of course, you need to know where to click in the image in order to produce the best effect. In this case, for example, the fender here is what we would call black, but it's actually a very dark shade of grey. If I click on that area with black eyedropper, I'll lose considerable information within the image. It's important to click on an area that actually should be a very dark black in order to produce the best results.
We can similarly adjust the white point by clicking on the white point eyedropper and then clicking an area of the image that should be perfectly white. In this case there are some specular highlights, reflections from the sun on the chrome. And so if I click there, you'll see probably no effect whatsoever. Probably just a very subtle color shift, if anything. Of course if I were to click in a different area of the image, I would produce a different result. Clicking on a shade of grey that's very very nearly white for example, as with the stripe here, produces a more obvious effect.
Although, in this case, that would cause a loss of detail in some of the other areas that were white. So in a case like this, I would click on the specular highlight or another area of the image that should be absolutely pure white. The gray eyedropper is a little bit different. Like the black and white eyedroppers, it does affect the overall color, but it doesn't change the tonality of the area that you click on. I could simply click on the grey eye dropper to make it active, and then click within the image. However, I want to demonstrate the ability to change the target value for each of these eye droppers.
If you double-click on any of the eye droppers, the color picker will appear. Here, you can specify a new target value for that eye dropper. For example, in this case maybe I want to apply a little bit of a yellow tint to the image, just to apply a golden quality to the light. I'll go ahead and choose some shade of yellow that's fairly neutral and then click okay. Photoshop will then ask if I want to save the new target colors I've defined as the defaults for curves. I'll go ahead and click no, because generally I would want to use the grey eye dropper to define a neutral point in the image.
Clicking no, I'm ready to click with my grey eye dropper inside the image in order to neutralize that value to the particular shade of grey, in this case a slightly yellow tinted shade of grey. That I've defined. So I'll click on something that should be neutral and as you can see I've applied a slight yellow tint to the image. Of course, normally I would use this to neutralize any color cast rather than adding my own color cast, so I'll go ahead and double-click on the gray eye dropper to bring up the color picker once again.
I'll then reset the values back to the defaults for the gray eye dropper. That happens to be a perfectly neutral middle gray, which has RGB values of 128 for all three numbers. With that established, I can click Okay, and once again there's no need to change the default values because gray already was the default. So I'll simply click No. Now if I click anywhere in the image, the pixle that I click on will be amde perfectly neutral gray. In the chrome here I can see the sky reflected so it's relatively blue. In order to compensate for a blue color cast we would need to shift the image toward yellow.
So clicking on an area of the image that's blue will apply a very strong shift toward yellow within the image. Clicking on something that's yellow will apply a blue shift. Clicking on something that's green will apply a relatively magenta shift, and clicking on something that should be perfectly neutral will cause the image to be adjusted so that area is in fact a neutral gray. This makes the gray eye dropper a great tool for quickly neutralizing any color cast in an image. I actually prefer not to use the eyedropper controls in curves under most circumstances.
In fact, I almost never use the black and white eyedroppers. And only occasionally use the grey eyedropper. I generally find that I can apply an adjustment faster by working manually with curves. And using my eye to evaluate the result. Part of the problem with the eyedropper controls, of course, is that you may end up clicking in a number of different areas of the image, searching for just the right place to click on, when you could have more easily applied the adjustment directly. Having said that, the eyedroppers can prove incredibly helpful in certain situations.
Especially when a particular object or area in the image must be perfectly neutral gray.
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