DOF is dependent on many things..... the aperture, the lens' focal length, the sensor size and the focus point.
For instance, let's say you take your D3100, with the lens set at 50mm. You set the aperture at f/8. What's the DOF? No way to tell without knowing the fourth.... the focus point.
So, let's say you focus the lens at 3 feet. Now the DOF can be calculated.....I come up with 2'-10" to 3'-3". That's only 5" of DOF. If you change nothing but the focus point, the total DOF will change. Let's say, go to 10'. Now you'll get 8'-1" to 13'-2" in focus.... a DOF of more than 5 feet.
Going back to the 3' focus point, changing the aperture to f/16 will increase the DOF to 2"-8" to 3'-6", for a total of 10", not just 5". Stopping down to f/32 will increase it to 2'-4" to 4'-2" (20" total).