  I study the importance of the accurate calibration of photometric systems in
order to produce meaningful comparisons between the observed colors +
magnitudes and model SEDs. Possible sources of errors are discussed and two
examples are analyzed. I show that well-calibrated Tycho-2 photometry is stable
and precise enough for such comparisons. On the contrary, the available
calibrations for Johnson UBV photometry yield relative large systematic errors,
which has prompted me to develop a new, more precise calibration. The
advantages of multicolor photometry over the standard single-color + magnitude
diagrams for the derivation of physical properties of stars (elimination of
degeneracies, inclusion of multiple parameters, avoidance of linearizing
approximations, possibility of a more precise treatment of errors) are
discussed through the use of CHORIZOS, a code developed specifically for this
purpose.
