There are two ways to access the Curves window, but only one of these ways will create a non-destructive layer that you can go back to and re-edit. The other method is most recommended for a quick edit to a photo you don't plan on saving for archival purposes. To access the Curves from this area, go up to the Image menu, then Adjustments and in there you'll see Curves. The other method is a much more recommended method and is accessed by looking at the bottom of the Layers window and looking for the Adjustment Layer pop-out menu. It's a small black and white circle icon and once clicked, you have all the same options as in the Image -> Adjustment menu, only these will all make a new layer that only affects the layers below them, thus preserving the actual image and allowing you to modify the adjustment layers at any point in the editing process.
Once brought up for the first time, the Curves window can be a bit intimidating in the sense that there aren't many intuitive clues to get you going. Luckily Adobe Photoshop CS3 has helped that out just a bit. One of the big new features is now you can see the histogram behind the curve. The histogram is a measurement of the lights and shadows in your photo. At the left are you shadows, in the middle the midtones and at the right are the highlights. Laid on top of the histogram is a diagonal line extending from the bottom-left to the top-right. The bottom-left of the line represents the darkest shadows and the top-right are the brightest highlights. You can drag either one of those points and you'll see the effect that the points have on that particular area. The magic comes in however, when you create new points. To do so merely click anywhere on the line and a new point appears! You can manipulate this new point by dragging it in any direction and then creating another point to create some new contrast. You can theoretically create as many points as you want, but as a rule of thumb, it's best to keep the curve moving from bottom-right to top-left. Too many interruptions from this motion will create some unwanted effects.Another new feature in CS3 are the Curve presets. You'll notice at the top of the window there's a Preset drop-down box that gives you some predefined curves that you can use to, if nothing else, give you a better idea of how the curves work when certain points are applied to certain places.
Now that you have a pretty decent idea about how to control the overall tones of the image, what about color? Located right under the Presets, you'll see another drop-down box labeled Channel. From here you can select either the Red, Green or Blue channels, each bringing up their own Curve window. This gives you ultimate control over how each different channel effects the entire overall image.Rather than blindly modifying the Curves from a brand new image, I find it's easiest to hit the Auto button and begin modifying from there, as needed. This can save quite a bit of time, especially if you have multiple images you'll be editing. The best part is that with Adjustment Layers combined with their Layer Masks, you can always add more Curves to tweak individual aspects of an image. You can see from this example how the Auto feature created much more contrast in this image just by tweaking the histograms for each channel with just one mouse-click. From here I typically OK that window and begin using new Curve Adjustment Layers to make any further changes or modifications.
You should have plenty of information now to help you get started taking full advantage of Curves. Just remember, there is a bit of a learning curve in understanding how to manipulate the curves. Keep practicing and one day it will click!
