Zoom will also affect DOF, the more you zoom in on a subject the shallower the DOF will be.
Its just a case of practising, set up a few staged shots and try different appertures and have a look at the effect.
BTW I was always told there is no such thing as a stupid questions, only stupid answers !!!!
If you want to edit any of my images to demonstrate points/techniques to either myself or others please feel free to do so.
Still learning with Canon 40D, 28-135mm, 100-400mm L and Elements 6.
Any Advice greatfully received.