200mm, 1/1250sec. at f/8.0
Curves is a powerful but complicated tool that is available in more advanced photo editing software like Photoshop or the GIMP. It deserves a thorough, well illustrated, and well thought-out explanation, so I'm not going to aspire to do that here. What I do want to do, though, is bring it to your attention and show you how it helped me greatly enhance this recent photograph. Then, I'll point you to some well written tutorials where you can learn about it in more detail.
I'm going to oversimplify things a bit and say that curves is a tool for adding contrast to your images, as well as controlling the relative brightness or darkness of different parts of your image.
Here's what the tool looks like in Photoshop. Understanding the real meaning of this graph is valuable, but it's a hard place to start--it's easier to start with a more superficial but practical understanding.
In the curves tool dialog, you can click and hold anywhere along the line, and drag up or down to add a gentle bend to that part of the line, creating a "curve". The simplest application of this tool is to create a slight "S" curve which will add some nice contrast to your image.
(The curves screenshots in the rest of this post are from Lightroom, which is where I normally use the tool.)
Hehe, that's a great photo. Jess took it; you can read about it here if you're interested.
This basic contrast is great, but curves may be overkill for just adding a little contrast to your image. A simpler photo editor will usually have a contrast slider which may be just as effective and easier to use.
Curves becomes a much more exciting tool when you think of it as a way to control the brightness of different tonal ranges in your image.
In my morning outing to Hendry's Beach that I posted about a few weeks ago, I took the photo at the beginning of this post. I'm almost always shooting wide-angle at the beach, but this was a moment where I was glad to have a 55-200mm zoom lens in my bag.
Take a look at these before-and-after images, showing the original image and the tone curve that I applied.
Using the curves tool, I essentially told it, "make just the 'light' part of the clouds a lot lighter, and make the 'dark' areas a little darker". This creates a much more dramatic and higher contrast image.
Both Lightroom and Photoshop also provide a great intuitive tool for working with curves. It allows you to click anywhere on the image, and adjust the part of the curve which corresponds to the point in the image where you clicked.
I'm tempted to try and explain the curves tool more, but I know I won't have time to do it justice. If you're serious about editing, you should look into curves. Take a look at these great tutorials, and I'll do my best to answer any questions you leave in the comments.
I have to admit I haven't fully read through this one, but it appears to be very thorough and well illustrated, and I'm looking forward to reading it and maybe picking up a few new things. It goes to show just how much background is required in understanding this tool.
This one is from one of my favorite sites for tutorials, cambridgeincolour.com. It's a little shorter and more to the point, but also more technical. You may be able to get going quickly with this one without completely understanding it, then re-read it over time and gradually build your understanding.