But now, let's assume you want to crop an image, and still bring it up to 8x10. Let's assume you want to crop a 5x7 area and bring that up to 8x10 inches. How would you figure out what camera you'd want?

Again, it's math. If the picture is ultimately going to be 10 inches across, printed at 220DPI, you're going to want to need at least 2,200 pixels across the image. Likewise, you're also going to need to have 8 inches times 220 dots, or 1,760 dots down the image.

But you want to scale up that 5x7 area into the 8x10 image. In a sense, you need to have a camera that can take considerably more than 3 megapixel images because the usable picture data is only in the middle. In fact, about 50% or so of the actual picture will be wasted. Calculating it out, 7 goes into 10 about 1.5 times. 5 goes into 8 about 1.6 times.

So, rather than the 2,200 pixels across the image and 1,760 down, you really need 3,300 pixels across (that's 2,200 times 1.5) and 2,816 pixels down (which is 1,750 times 1.6). Multiplying out 3,300 times 2,816, we get a whopping 9,292,800 pixels, or what is really a 10 megapixel camera.

This is why professional photographers and photographic artists are still using film, or are using very high-end cameras. In your case, depending on your quality requirements, you could cheat by reducing the print quality and seeing what turns out right.