Channeling Punky Brewster.
(All photos taken with Nikon 85mm, f/1.8 lens.)
So let's talk about depth of field for a quick minute. In trying to figure out what f stop to use, you need to understand what depth of field is. I used an f stop of 5.6 for both of these photos.
You can easily see bokeh (or out of focus) area in the first photo. In the second photo, notice how the car behind the girls is not really that out of focus. For the first photo, I was only a few feet away from Em. In the second photo, I was almost all the way across the street from the girls.
As you distance yourself from your subjects, the depth of field changes. In my example, more of the photo is going to be "in focus" as that distance increases. Make sense?