And just to add to what Ralph and Les have said--if you want to use two different apertures to control diffusion it's easiest usually to do this as a multiple exposure. You could, say, make 25% of the exposure at a wide aperture or with a soft focus or diffusion attachment and 75% of the exposure at a small aperture.

You see this a lot in product advertisements and sometimes with food shots, where the object is sharp but has a kind of a glow, but you could also use it in a landscape or anything where the subject isn't moving (unless you want to record the motion, like Les does with his multiple exposure technique).