One part of the exposure triangle, as we discussed, is aperture. Aperture controls our depth of field but not all depth of fields are the same. The amount of depth of field is influenced by many things including the type of lens, it's focal length, the focal plane and so much more than we can get into in this post. So to keep things simple to understand we will discuss depth of field in how it relates to focal distance and its relative focal plane.
For this discussion we will refrain from discussing specific lenses or indicating specific aperture settings. Those numbers will vary widely depending on many variables anyway. I will simply keep it to a shallow depth of field based on the different focal distances used in these examples. Shallow depth of field is obtained by using a wide aperture and following the basic rules of exposure.
I also want to establish the definition of focal plane and depth of field. Focal plane refers to the principal where the point of sharpest focus on your subject extends outward, from the subject, parallel to the camera sensor. Therefore something beside your subject and parallel to your sensor will be in focus as it sits in line with the focal plane (the illustration above shows this principal) while something in front of or behind the subject will start to loose focus. That area of relative sharpness is your depth of field.
This area of sharpness degrades in front of and behind the focal plane and the amount of perceived degradation is determined by the physical properties of the lens, distance of the subject from the camera and distance of adjoining objects. However there is a certain amount of sharpness immediately in the vicinity of the focal plane and it is this area of relative sharpness that becomes the depth of field (this is illustrated by the red area in the following examples).