  We study the effects of applying observational techniques to derive the
properties of simulated galaxies, with the aim of making an unbiased comparison
between observations and simulations. For our study, we used fifteen galaxies
simulated in a cosmological context using three different feedback and chemical
enrichment models, and compared their z=0 properties with data from the Sloan
Digital Sky Survey (SDSS). We show that the physical properties obtained
directly from the simulations without post-processing can be very different to
those obtained mimicking observational techniques. In order to provide
simulators a way to reliably compare their galaxies with SDSS data, for each
physical property that we studied - colours, magnitudes, gas and stellar
metallicities, mean stellar ages and star formation rates - we give scaling
relations that can be easily applied to the values extracted from the
simulations. These scalings have in general a high correlation, except for the
galaxy mean stellar ages and gas oxygen metallicities. Our simulated galaxies
are photometrically similar to galaxies in the blue cloud/green valley, but in
general they appear older, passive and with lower metal content compared to
most of the spirals in SDSS. As a careful assessment of the
agreement/disagreement with observations is the primary test of the baryonic
physics implemented in hydrodynamical codes, our study shows that considering
the observational biases in the derivation of the galaxies' properties is of
fundamental importance to decide on the failure/success of a galaxy formation
model.
