The smaller the number the smaller the DOF. The exact amount of DOF will very depending on focal lenght. For example, f 3.5 at 28 will still look like it has a pretty large DOF, while the same f-stop at 90 will produce a much shorter DOF. Also distance from subject effects it. Actually i'm not sure if it really does or not, it may just appear to because the smaller objects look more realistic blurred