Tweak the Colors in Digital Pics

You're better-equipped than the National Aeronautics and Space Administration. Think about it: Until just a few years ago, satellites that took pictures of Earth were analyzed using multimillion-dollar false-color image applications that had less than a tenth the power of a $100 program like Jasc Paint Shop Pro or Adobe Photoshop.

That's impressive. But is most of your better-than-NASA image editor going to waste? This week, let's dig into one of the most important--but poorly understood--features in any image editor: the Curves tool.

So what are Curves? Curves are, simply put, graphs that let you control the brightness of individual color channels in your picture. You can make the blues brighter or darker, for instance, while leaving the reds unchanged. And more than that, you can brighten the blues only in the shadows, while darkening the blue highlights. The power of the Curves tool is incredible, but getting good results take a little practice.

In Paint Shop Pro, you can find the Curves tool by choosing Adjust, Brightness and Contrast, Curves. When you first open the tool, pay attention to two things:

The input light level is the value of light in a given pixel in the original picture. The output light level is the new value of the pixel after you change the graph. Before you fiddle with anything, the graph is a diagonal line, which means input equals output, and nothing is changed.

But think about what happens if you grab the diagonal line in the middle and drag it up so it bulges like a mountain. If you look at the graph, you can see that you've "mapped" input light levels to higher output levels. The end result? The picture gets brighter. Try it, making sure to click the Auto Proof button so you can see your changes in the picture itself as they happen in the graph. (The Auto Proof button is in the center of the screen; it's the eye with the lock on it.)

Hopefully, all this starts to become clear after you experiment with making curves in the graph for a little while. So let's apply what we've learned to a real photograph. For this example, I'll work with a lighthouse that I photographed on Cayman Brac last year.

It's not a bad picture, but I'd like to make the sky bluer and pull some of the red out of the structure, making it look perhaps a bit more brown. Sounds like a job for Curves!

Open the Curves tool by choosing Adjust, Brightness and Contrast, Curves. Let's start with the sky--we want to punch up the blue a bit. Since the sky is a highlight in the picture, which means it's got some of the brightest pixels in the scene, we know that we don't need to increase blue everywhere in the picture. Instead, all we want to do is affect the highlights.

To do that, first choose Blue from the channels menu, and then click in the middle of the diagonal line--but don't drag it anywhere. You've added a control point, and the curve will behave as if you stuck a thumbtack there. Now grab the curve higher up than the control point and pull it upward so the sky brightens.

Now for the other half of the project, stamping some of the red out of the lighthouse. Change to the red channel. The red we want to remove is in the shadows, which lie at the bottom of the curve. So add a control point in the middle and drag the bottom of the curve down. You should see the lighthouse change accordingly.

If you played along at home, your final image should look something like mine--and if it does, you now know how to precisely control the colors throughout your pictures.

Q&A: Comparing Pixels to Film

I'm looking to make the move from film to digital. How many megapixels would a digital camera need to have in order for its resolution to be equivalent to a 35mm SLR? 6.3? 8? 11? More? My current thinking is to begin by purchasing a high-resolution scanner and getting a digital camera later. What kind of quality do I need so I can't tell the difference between a digitized 35mm slide and a real 35mm slide when projected on a screen?

--Andrew Apicos, San Diego

You've asked the million-dollar question, Andrew. Everyone always wants to know how many pixels it takes to match film.

Unfortunately, the answer isn't easy to pin down. I have to agree with all of the numbers you put out. It simply depends. If you shoot ISO 400 film with your 35mm camera, then 6 megapixels is probably enough to reach parity. If you shoot on ISO 25 slide film, you're going to need more like 12 megapixels in your digital camera to get a similar result.

And don't forget that the quality of your photographs--both digital and film--is strongly influenced by the available lighting, the shutter speed, and other factors.

The simplest answer is that 6 to 8 megapixels is a good starting point for matching the quality of film. Higher resolutions allow you to make progressively bigger prints or perform more aggressive cropping and still get a high-quality print.

Comparing digital and film slides has similar complexities. If the size of the projected slides is small enough, the digital resolution is almost irrelevant. The longer the throw of the projector, though, the more resolution becomes an issue: Obviously, the larger the image, the more obvious digital artifacts and jagged pixels will become.

But all that's theoretical: It's simply not possible to project a full-resolution 12-megapixel picture anyway. That's because even high-end HDTV projectors--models that cost $10,000 to $20,000--deliver a native resolution of 1280 by 720 pixels. That sounds low, but the results can be stunning.