  Synthetic photometry is a great tool for studying globular clusters,
especially for understanding the nature of their multiple populations. Our goal
is to quantify the errors on synthetic photometry that are caused by
uncertainties on stellar and observational/calibration parameters. These errors
can be taken into account when building synthetic color-magnitude diagrams
(CMDs) that are to be compared to observed CMDs. We have computed atmosphere
models and synthetic spectra for two stars, Pollux and Procyon, that have
stellar parameters typical of turn-off and bottom red giant branch stars in
globular clusters. We then varied the effective temperature, surface gravity,
microturbulence, the carbon, nitrogen, and oxygen abundances, and [Fe/H]. We
quantified the effect on synthetic photometry in the following filters: Johnson
UBVRI and HST F275W, F336W, F410M, F438W, F555W, F606W, and F814W. We estimated
the effects of extinction, atmospheric correction, and of the Vega reference
spectrum on the resulting photometry. We tested the ability of our models to
reproduce the observed spectral energy distribution and observed photometry of
the two stars. We show that variations are generally stronger in blue filters.
Dispersions on synthetic colors due to uncertainties on stellar parameters vary
between less than 0.01 and to 0.04 magnitude, depending on the choice of
filters. Uncertainties on the zero points, the extinction law, or the
atmospheric correction affect the resulting colors at a level of a few 0.01
magnitudes in a systematic way. The models reproduce the flux-calibrated
spectral energy distribution of both stars well. Comparison between synthetic
and observed UBVRI photometry shows a variable degree of (dis)agreement. The
observed differences likely indicate that different calibration processes are
performed to obtain respectively observed and synthetic photometry.
