In any case, my screen is 13.25 inches long as displays 1,024 dots in that space. Dividing 1,024 by 13.25 tells us that my screen displays 77.28 dots for every inch of screen space. In effect, my screen is 77.28 dots per inch.

With me so far?

Printers, these days, print at resolutions above 300 dots per inch, although I've found that often, anything about 220 or so seems fine. For the purpose of our discussion, though, let's use 220 dots per inch as a minimum and relate that to print size.

Here's another math quiz. If you want to print out a 5x7 picture, how many megapixels do you need?

Start with the dots per inch. At 220DPI, there are 1,100 dots in five inches (220x5). Likewise, there are 1,540 dots in seven inches because 7 times 220 equals 1,540. Multiply 1,100 times 1,540 and you get 1,694,000 pixels in the image.

The next step is easy. 1,694,000 pixels is 1.6 megapixels. To print out a nice 5x7 image, you'd need a camera capable of 1.6 megapixels, which, in practice, means you'll get a 2 megapixel camera.

If this stuff still doesn't make sense, re-read the above paragraphs a few times. It's really the dots that fit in the space, multiplied out.

Thinking about megapixels
Now that you know about megapixels and a little about printing, let's learn how to think about what this means for your camera and your usage.

Here are some simple rules of thumb:

• If all you're ever, ever going to do is put the pictures on your Web site, on eBay, or email them to friends, you really don't need much more than a 1 megapixel camera.

• If you're going to print your images, you need to determine the largest size you want to print, then do the math.

• If you're going to want to crop your image, or extract a portion of the picture to print, you'll need to do the math for the area you want to crop.

And it all was so clear until now, huh? Let's examine those last two points in further detail.

I'm a photographic artist and I like to print out my images, frame them, and put them on my wall. My pictures are usually 8x10, but they often go as high as 13x17 and as small as 5x7. If you're like me, you'll want to have a camera that's up there in the megapixels -- or wait and keep using film.

If you're likely to only want to print 5x7 images, then, as we showed above, a 2 megapixel camera is fine. If you're likely to want to print 8x10 images, then a 3 megapixel camera is fine.

If you're an artist or a professional, and you're going to twiddling with the dots per inch on your printer for maximum quality, you will also probably want a higher-end camera.

I'm already way above my word-count limit for this article, so you'll have to wait until next week to learn about cropping and how to plan for it with your first digital camera.

Stay tuned.