  The ability of new instruments for providing accurate inferences of vector
magnetic fields and line-of-sight velocities of the solar plasma depends a
great deal on the sensitivity to these physical quantities of the spectral
lines chosen to be measured. Recently, doubts have been raised about visible
Stokes profiles to provide a clear distinction between weak fields and strong
ones filling a small fraction of the observed area. The goal of this paper is
to give qualitative and quantitative arguments that help in settling the debate
since several instruments that employ visible lines are either operating or
planned for the near future. The sensitivity of the Stokes profiles is
calculated through the response functions (e.g. Ruiz Cobo & Del Toro Iniesta,
1994). Both theoretical and empirical evidences are gathered in favor of the
reliability of visible Stokes profiles. The response functions are used as well
for estimating the uncertainties in the physical quantities due to noise in the
observations. A useful formula has been derived that takes into account the
measurement technique (number of polarization measurements, polarimetric
efficiencies, number of wavelength samples), the model assumptions (number of
free parameters, the filling factor), and the radiative transfer (response
functions). We conclude that a scenario with a weak magnetic field can
reasonably be distinguished with visible lines from another with a strong field
but a similar Stokes $V$ amplitude, provided that the Milne-Eddington
approximation is good enough to describe the solar atmosphere and that the
polarization signal is at least three or four times larger than the typical rms
noise of $ 10^{-3} I_{\rm c}$ reached in the observations.
