This is the curves window. It lets you map the a brightness level on the original photograph (the input) to a new brighness level on the adjusted photograph (the output). The horizontal axis is the input level and the vertical axis is the output level. The straight diagonal line shows that the output equals the input. In other words, the adjusted photograph will look just like the original photograph. By changing the shape of the diagonal line, you change the output level for a give input. By default the gridlines are set at 25%. To change this to 10% as shown, press and hold the Alt key (Option key on the Mac) and left-click anywhere on the grid.
We'll start by putting three points on the line. You do this by clicking along the line at the locations where you want the point. These three points correspond to the highlights, midtones, and shadows. We can adjust these individually without affecting the others. The solid black point shows that the highlight is selected. The values for the highlight point is shown in the Input: and Output: fields. The values are RGB values. In this case it says that any pixel with an original value of 192 will have a new value of 192. We can adjust this point by ether entering a new value or by dragging the point on the line.
Now every pixel with a value of 192 will have a value of 218 in the output image. This has the effect of making the highlights brighter. Now we'll adjust the bottom point.
The output of 64 is now 49. This has the effect of making the shadows darker. This is called an S-curve. This curve will increase contrast. To decrease contrast pull the curve in the other direction (i.e. lighten the shadow and reduce highlights) You'll find that you use S-curves quite often.
You can add as many points on the line as you like and pull them in any direction you like. Curves allows to you make precise, complex adjustments.
Let's try curves adjustment on an example. Instead of using levels, we'll use curves to adjust this photograph
I'll first start pulling up the midpoint. This will increase the overall brightness of the image.
The image already looks pretty good as is. But we can make more adjustments. Let's darken the shadows. Add a shadow point and pull it down.
Whoa! What happened? Not only did it make the photograph too dark, it also blew out the sky in the top left corner.
Here's how the curve changed. As you can see, when the shadowpoint was dragged down, it caused the top end to curve upward. This is what caused the sky to blow out. To compensate let's to drag down the highlights.
Now change the shadows.
As you can see, you can add as many points as you like to precisely control the shape of the curve.
You can also use curves to add subtle color to your black and white images.