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One of the best ways to improve your confidence when it comes to applying a curves adjustment is to learn how to read a curve. By understanding what effect a given curve will have on an image, you'll gain the ability to anticipate the sort of adjustment that is necessary in order to achieve a desired result in an image using curves. Curves utilizes the concept of before and after. It refers to those values as input and output as you can see on the adjustments panel. By before and after values, what I mean is that we're changing the luminence of pixels within the image. In other words, we're adjusting overall brightness and contrast. The gradient along the bottom of the curve is our before value. For example, if we take a look at the middle of the gradient, we'll find a luminence value of middle gray.
Now this doesn't mean that the pixel that relate to this value are actually gray. They could be any color of the rainbow. They simply have a luminescence that is equal to middle gray. If we go directly upward from this gradients until we intersect the curve, and then move directly to the left we'll see the after value. This is shown on the gradient along the left side of the curve. Now as you can see at the moment our curve is comprised of a straight line going from the bottom left corner to the top right corner. As such, all of the before and after values match up with each other. Pixels that had illuminates of white are still going to be white. Pixels that had illuminates of black are still going to be black. And for every shade of gray in between, again representing luminance values, the before and after values will be the same.
In other words, the input and output values will match each other. If we raise the curve, we'll be brightening the image because now the after values are all brighter than the before values. For example, taking a look at middle gray, I started the center of the gradient, moved vertically until I intersect with the curve and as you can see my after value is brighter than middle gray. By contrast, if we lower the curve, we'll be darkening the image. Because now the after values are all darker than the before values.
We can also adjust contrast with curves. If I increase the steepness of the curve, there will be more contrast in the image. More steepness equals more contrast. If I reduce the steepness of the curve, then I'll be reducing contrast. Less steepness equals less contrast. And I can even take this to an extreme by making the curve go downhill, which will produce an inverted version of the image. As you can see, the fundamental behavior of curves is relatively straightforward.
The steepness of the curve relates to contrast and the relative height of the curve at a given point relates to brightness. By understanding the basic concepts involved you'll soon be able to master the use of curves to optimize your photographic images.
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