  We test whether we can predict optical spectra from deep-field photometry of
distant galaxies. Our goal is to perform a comparison in data space,
highlighting the differences between predicted and observed spectra. The Large
Early Galaxy Astrophysics Census (LEGA-C) provides high-quality optical spectra
of thousands of galaxies at redshift $0.6<z<1$. Broad-band photometry of the
same galaxies, drawn from the recent COSMOS2020 catalog, is used to predict the
optical spectra with the spectral energy distribution (SED) fitting code
Prospector and the MILES stellar library. The observed and predicted spectra
are compared in terms of two age and metallicity-sensitive absorption features
(H$\delta_\mathrm{A}$ and Fe4383). The global bimodality of star-forming and
quiescent galaxies in photometric space is recovered with the model spectra.
But the presence of a systematic offset in the Fe4383 line strength and the
weak correlation between the observed and modeled line strength imply that
accurate age or metallicity determinations cannot be inferred from photometry
alone. For now we caution that photometry-based estimates of stellar population
properties are determined mostly by the modeling approach and not the physical
properties of galaxies, even when using the highest-quality photometric
datasets and state-of-the-art fitting techniques. When exploring a new physical
parameter space (i.e. redshift or galaxy mass) high-quality spectroscopy is
always needed to inform the analysis of photometry.
