Illustrated Depth of Field

Changing the aperture of a lens changes depth of field (DOF). The dotted black lines show how well the lens can focus the image. As an object moves away from the best focus position (where the dotted lines cross), object details move into a wider area of the cone (three dimensional imagination). The wider the spread of the cone, the more the image blurs into the surroundings.

Modified and simplified from edmundoptics.com