D is for Depth of Field
Depth of field describes the distance in front of and behind an object that is in focus at the same time that the main object is in focus.
We are, in general, familiar with this. If something is close to us and we focus on it, things in the background will be out of focus. Or, when we focus on something in the distance, things in the foreground become blurry.
Depth of field is slightly more specific in that it actually quantifies the space in front or behind an object that will be in focus. The construction of the lens has a strong effect on this. However, depth of focus can also be affected by the size of the aperture. As an aperture gets smaller, the depth of focus increases.
Depth of field can be used to create interesting photographic effects. It’s also important to control depth of field when photographing objects for professional publication or study.