Depth of Field

The depth of field in photography is the part of an image that's in focus. It varies with the aperture (size of lens opening). If you have the focal length and aperture of the lens you're using and the distance to your subject as well as the size of your sensor, you can calculate an approximate depth of field.

In the picture above I can use the following values for the image above to calculate the depth of field:

camera sensor focal multiplier effect=1.6X
aperture=F5.6
focal length=77mm
focus distance to subject=15m

In the photograph above the depth of field is calculated to be nearly infinite, which, as you can see, is the case. The image appears clear through the background.