Megapixels and How They Affect Photos

Camera manufacturers are fond of advertising cameras by the number of megapixels they have. But what exactly is a megapixel and how does it affect photos?
A megapixel is 1 million pixels. Pixels are small squares that are put together like pieces of a puzzle or mosaic to create your photographs. The resolution of your image will be determined in large part by how many of these tiny squares are packed together in a small space.
An 8-megapixel camera (8MP) would have roughly eight million tiny squares of information per inch while a camera phone at 1.5 megapixels (1.5MP) would only have one and a half millions squares of information in an inch.

Simply put, the more information the better. The more information squeezed into an area, the better our eyes blend the edges together to create a complete image. If too little information is available the eye will notice the jagged edges of the pixels where they meet, just as you see the individual squares of mosaic tile designs. The accepted "standard" for printing images is currently 300dpi (dots per inch). While dots per inch aren't technically the same as pixels per inch the difference won't affect you in your day to day photo taking/printing.

To figure out how much information you need for a specific print size all you need do is multiply the print size by the resolution desired. For example, with the 300dpi rule in mind, to print an 8x10 photo you would need 2400 pixels by 3000 pixels of information. If you were displaying an image on the internet (where 72 pixels per inch is acceptable) you would only need 576 pixels by 720 pixels.

Each camera displays data in slightly different ratios but there are some "rules of thumb" you can follow. Decide what the largest size image you will want to print. For most people, this will be an 8x10 image. Determine the number of pixels needed for a 300dpi print (2400x3000 for an 8x10). Next multiply the two-pixel dimensions together. For an 8x10 this comes out to 7.2 million pixels or 7.2 megapixels. This is the preferred number of MP you need if an 8x10 print is the largest you are likely to print.

4x6 2.1MP
5x7 3.1MP
8x10 7.2MP