Depth of field refers to how far objects can be from the focal point before they start to become blurry.
So in a shallow depth of field will blur out nearly everything but the objects that are on the same focal plane.
This is most evident in portraiture, where the distance from the subject’s eyes to her nose might be enough to demonstrate the effect of a shallow DOF:
How does depth of field relate to aperture?
Wide apertures, e.g. f/1.2, f/1.8, result in shallow depth of fields. As we learned in the aperture chapter, the extra stops of light provided by wide aperture is the domain of specialized and/or expensive lenses. This is true for DOF.
When do we want a shallow depth-of-field?
It can be used strategically to isolate the subject from the details in a distracting background.
When do we want a deep depth-of-field?
As we saw in the example portrait, a shallow DOF isn’t forgiving if your focal point is slightly off. An extremely shallow DOF, in which someone’s various body parts are in and out of focus in the same portrait, may not be appealing to every client.
In scenes where you want as much in focus as possible, such as landscape photography, it’s best to stop down (i.e. increase the f-number) the aperture. Of course, this is usually only viable for well-lighted scenes (or extremely long shutter speeds).