understanding depth of field

How to Define your Subject using Depth of Field

DOF is the distance between the nearest and farthest objects in a photograph that appear reasonably sharp. Think of this area as the focal plane as well. Lenses can precisely focus on a given point. However, anything closer or further away from that point begins to leave the focal plane. This is how you get the nice blurry backgrounds you see in some photographs or the tack sharpness in others.