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The S\'ersic law (SL) offers a versatile functional form for the structuralcharacterization of galaxies near and far. Whereas applying it to galaxies witha genuine SL luminosity distribution yields a robust determination of theS\'ersic exponent eta and effective surface brightness $\mu_{\rm eff}$, this isnot necessarily the case for galaxies whose surface brightness profiles (SBPs)appreciably deviate from the SL (eg, early-type galaxies with a depleted coreand nucleated dwarf ellipticals, or most late-type galaxies-LTGs). In thisgeneral case of "imperfect" SL profiles, the best-fitting solution maysignificantly depend on the radius (or surface brightness) interval fit andcorrections for point spread function (PSF) convolution effects. Suchuncertainties may then affect, in a non-easily predictable manner, automatedstructural studies of galaxies. We present a fitting concept (iFIT) thatpermits a robust determination of the equivalent SL model for the general caseof galaxies with imperfect SL profiles. iFIT has been extensively tested onsynthetic data with a S\'ersic index 0.3<${\eta}$<4.2 and an effective radius1<$\rm{R}_{eff}$ (arcs)<20. Applied to non PSF-convolved data, iFIT can inferthe S\'ersic exponent eta with an absolute error of <0.2 even for shallow SBPs.As for PSF-degraded data, iFIT can recover the input SL model parameters with asatisfactorily accuracy almost over the entire considered parameter space aslong as FWHM(PSF)<$\rm{R}_{eff}$. Tests indicate that iFIT shows littlesensitivity on PSF corrections and the SBP limiting surface brightness, andthat subtraction of the best-fitting SL model in two different bands yields agood match to the observed radial color profile. The publicly available iFIToffers an efficient tool for the non-supervised structural characterization oflarge galaxy samples, as those expected to become available with Euclid andLSST.

A new fitting concept for the robust determination of S\'ersic model parameters