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There are many articles online, and in various books on photography, that try to explain how many mega pixels you need, but most fail to adequately explain this, let alone tell you the most important and fundamental bit info. Why?

The first thing you need to understand is a little bit of biology, specifically human biology. The average human eye sees much like a digital camera sees. On the back of the eye there are many small receptors that capture light rays and convert them to electric signals that our brain interprets as an image, both the shade (darkness) and the color. The number of these receptors (Rods and Cones) determines how we see color, and how we see a dot matrix image (such as a TV screen, or a digital image). Some people have different numbers of these receptors or have conditions which have damaged them, or that have other wise interfered with the whole process (Color blindness, etc), but for the most part, the average human sees about the same.

If we were to take a one inch by one inch square piece of paper and put 1 dot on it with a pencil, we would have a picture. Granted it's not a particularly good picture, but it is a picture. It is defined as a 1 dpi image. DPI stands for Dots Per Inch. This is interchangeable, in this realm with PPI or Pixels Per Inch. Now. If we place 99 more dots on that square, we will have a total of 100 dots in our picture. Some of the dots are black, some are red, some are blue. But we still have 100, and the square is still 1 inch by 1 inch, so, our image is now a 100 dpi image. At this point, we can still see, at viewing distance, or arms length, ever dot on that image fairly easily. As we increase the dots that we place, filling in more of the square, we eventually will not be able to discern were one dot ends and another begins. For most people, this magic number is right around 275 dots on a 1 inch square, or 275 dpi. Now that does seem very magical, so we in the industry use the standard of 300 (both to make math easier and to account for those of us with more astute vision). So keep this number in mind. 300 dpi is the most your eye can discern.

Now we can properly answer the question of how much resolution is needed. Lets assume that, like most photo snappers, you take pictures that are going to printed at a mini lab (just like your film used to), at a size of 4x6. Remembering that our eye can see 300 dots per inch, and realizing that a pixel is a kind of dot, we can see that a 4x6 picture will require (4 inches

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