  An assessment on the capabilities of modern spectropolarimeters and
magnetographs is in order since most of our astrophysical results rely upon the
accuracy of the instrumentation and on the sensitivity of the observables to
variations of the sought physical parameters. A contribution to such an
assessment will be presented in this talk where emphasis will be made on the
use of the so-called response functions to gauge the probing capabilities of
spectral lines and on an analytical approach to estimate the uncertainties in
the results in terms of instrumental effects. The Imaging Magnetograph
eXperiment (IMaX) and the Polarimetric and Helioseismic Imager (PHI) will be
used as study cases.
