There are three factors that are going to determine the size of this depth of field: aperture, focus distance, and focal length.

Making the aperture (size) of the lens bigger, the angle at which the light is coming in and the angle at which light is being bent is made much steeper.

As a result, the trajectories of light fail to converge even more, andthe depth of field range is made smaller. For example, if you went from an f/5.6->f/2.8 aperture, the depth of field range would shrink, and vice versa.

change the distance of the subject, or the distance that you're focusing at. Again, this changes the angle at which light comes in - with longer distances you have less steep angles, and correspondingly the light converges closer, giving you more depth of field.

Instead of standing right up to your subject and using a very close focus distance, for example, you can stand further away and use a longer focus distance, giving you a larger depth of field range.

Focal length
is an attribute of the glass itself - the numbers themselves have no real meaning, but with longer focal length lenses the light is bent less sharply, and thus depth of field will be greater.

we utilize longer focal lengths when we have a narrow angle of view, or when we "zoom in", so by zooming in we get a smaller depth of field range.

Now, what do we want - a large depth of field that gets more things in focus, or a small depth of field where only some things are in focus?

Generally, things where you want a large depth of field are scenes like landscapes, or architecture, or macro.

In them , you want everything to be sharp and in focus, so you want to shoot at smaller apertures to get a large depth of field

Architecture with small depth of field - with small depth of field you don't get to see any details in the building
Macro with small depth of field - we don't get to see all the details in the image

However, shallow depth of field can have its uses too. Oftentimes, using a shallow depth of field helps us isolate a single subject, since everything else (distracting background, other image elements) will be blurred out.

Portraits
Flowers
Sports

The Portrait Project
The assignment is to combine all of the elements we've learned to do a portrait session with another student in the class.

Specifically, you want to focus on the following things:
Lighting (direction, intensity)
Vantage point (camera direction, setting, background)
Perspective (subject distance vs. focal length)
Depth of field
Directing the subject (poses, props, expression)