Here's a simple rule of thumb: the more pixels in a given space the more detailed the image.

You can see this clearly by comparing the little picture to the big one. They're the same image (well, Ok, I cropped out much of the flower pot in the expanded image) but because the little picture has all those dots in a tiny space, the picture looks much more like flowers and a lot less like a quilt of colored squares.

This is the essence of digital photography: your eye blurs dots that are really close together. And understanding the relationship between the number of dots and the quality of the image will make everything a lot more clear (pun intended).

What the heck is a megapixel?
Now that you understand what a pixel is, let's move on to megapixels. Technical, megapixel refers to a million pixels. Before you jump up and down and think that's a lot of pixels, let's see what that really means.

It's all about math. Time for a simple math quiz. Honestly, it's simple, so no freaking out. OK? OK.

If you have 1024 pixels across your screen and 768 pixels down, how many pixels do you have all together? Well, you multiply 1,024 times 768 and you get 786,432 pixels. That's not quite a million pixels, but it is close.

Here's observation number one: if you just want a picture that'll look good on your screen, and your screen is 1024x768, a 1-megapixel camera is just fine.

When the camera manufacturers describe their cameras by the number of megapixels, then, they're really describing the number of picture elements that make up the pictures the camera takes. And they're talking, generally, about pictures in a 1.3 to 1 ratio. In other words, for every one pixel down, there's 1.3 pixels across. This makes sense. There'd be no use for a camera that takes a picture a million pixels long, but only one high.

The relationship of pixels to printing
If all you want to do is make wallpaper for your desktop, you really don't need to know any more. But if you want to print your pictures, maybe put them on your wall, you'll need to know about how printing resolution is measured.

By the way, lots of you asked about printing pictures yourself vs. using a service bureau. I'll talk more about that in another piece, but the short form is I tend to like printing them myself. Details on how to do it right will be another week.

In order to understand the relationship of pixels to printing, we need to know another term: dots per inch (or DPI). If you think about it, DPI makes a lot of sense. It's the number of dots in a given inch of the image. Most computer screens display 72 to 100 dots per inch.

The monitor I'm using right now is a 17 inch display, measured diagonally. That means it's about 13.25 inches across the top and 10.5 inches down. You may remember something called the Pythagorean Theorem that helps you determine the length of the diagonal if you know the length of the two sides. I'm trying to avoid math class here as much as possible, so I won't bore you with all the details.