It is generally accepted that, if in collection near-field optical microsco
py the probe-sample coupling can be disregarded, a fiber probe can be consi
dered as a detector of the near-field intensity whose size can be accounted
for via an intensity transfer function. We show that, in general, this per
ception is wrong and it is impossible to introduce such a transfer function
for the detected signal. Instead, we introduce an amplitude coupling funct
ion that relates the near-field amplitude and the amplitude of a mode guide
d in a probe fiber toward a detector. Different experimental configurations
are considered with respect to the relation between near-field optical ima
ges and the corresponding intensity distributions. Our conclusions are supp
orted with numerical simulations and experimental results obtained by using
a photon scanning tunneling microscope with an uncoated fiber tip. (C) 199
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