  We discuss theoretically the concept of spatial resolution in near-field
scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17
(2009) 19969] which reported on the achievement of active tips made of a single
ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and
on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to
decrease steadily in size, we assimilate a nanodiamond-based tip to a
point-like single photon source and compare its ultimate resolution with that
offered by standard metal-coated aperture NSOM tips. We demonstrate both
classically and quantum mechanically that NSOM based on a point-like tip has a
resolving power that is only limited by the scan height over the imaged system
whereas the aperture-tip resolution depends critically on both the scan height
and aperture diameter. This is a consequence of the complex distribution of the
electromagnetic field around the aperture that tends to artificially duplicate
the imaged objects. We show that the point-like tip does not suffer from this
"squint" and that it rapidly approaches its ultimate resolution in the
near-field as soon its scan height falls below the distance between the two
nano-objects to be resolved.
