Originally Posted by gonzogolf
This would be an instance where you dont want to change your aperture, but rather lengthen your shutter speed. A longer exposure will prevent people who are moving through the subject to dissapear as they are not in one place long enough to be recorded.
This is correct, if you want a shallow dof and shoot at F4, by putting on your ND you will slow down the shutter speed. You can slow it down even more by moving to say F8 or F11, this will increase your dof but also give you even slower shutter speeds.
The slower the shutter speed the more chances you have to have people disappear from the picture.