Only the aperture was changed. This helps highlight the value of taking a series of photos with different aperture values, then comparing them. Background blur in each photo differs greatly. Adjusting the aperture allows you to make a difference in the distance-focus relationship. Also, the wider the focusing plane and the wider the portion of the image that appears in focus, the more blur there will be.
Depth of field is the area in which the image appears to be in focus.
This means that the smaller the aperture is, the deeper the depth of field will become. So, depth of field changes with aperture, the lens' focal length and shooting distance. The general rules are shown below. Taken with f16 2.3. There are interesting lenses available today that can be used to help control blur.