It is shown that shear-force imaging, as is commonly used for distance
regulation in scanning near-field optical microscopy, is not a reliab
le technique for accurate topographic measurements. This is because di
fferent materials experience different shear-force damping. Results of
the shear-force damping characteristics are presented for a number of
different materials, and some consequences of the different dampings
for different materials are demonstrated. It is also shown that there
are at least two distinct shear force damping mechanisms. Results of i
maging small conducting islands on a glass substrate show that the dam
ping characteristics depend on the islands' size. (C) 1996 American In
stitute of Physics.