I could spend an incredible amount of time writing about all the factors that make up depth of field. But so much has been written already that it would really be an act in futility. However what I can do is direct you to links that contain good information about circles of confusion, which they are and depth of field.
What is important and worth reading about and learning is why you want to use depth of field of your lenses for more creative photography. It is a very important part of your daily photography just like getting the focus point correct, and the exposure nailed.
The issue of Depth of Field (DOF) seems to cause a great deal of confusion. Every photographer understands it somewhat, but rarely with the rigor needed to explain it to others. In fact, to be completely honest, I’ve had to go to a couple of reference books to be sure that I get the explanations here technically correct.
The term “Depth of Field” describes the range in a photograph, from near to far, that appears to be in focus. Everyone knows that the more a lens is stopped down, the more depth of field there will be, (true). Most people also believe that wide angle lenses have more depth of field than telephoto lenses (false). Let’s see what the facts really are, and why.
Editor note: This article is a bit dated but most certainly answers the questions about depth of field. The concepts don’t change because a few years have gone by
There is also software for your PC, MAC, IPHONE, ANDROID PHONES, IPADS, ETC. that can really settle the whole issue for you. As you begin to use little programs like this you will begin to understand how your particular sensor sizes and lenses perform and you will eventually not have to rely upon them to set up your perfect shot.