  We study a class of metric-variation f(R) models that accelerates the
expansion without a cosmological constant and satisfies both cosmological and
solar-system tests in the small-field limit of the parameter space.
Solar-system tests alone place only weak bounds on these models, since the
additional scalar degree of freedom is locked to the high-curvature
general-relativistic prediction across more than 25 orders of magnitude in
density, out through the solar corona. This agreement requires that the
galactic halo be of sufficient extent to maintain the galaxy at high curvature
in the presence of the low-curvature cosmological background. If the galactic
halo and local environment in f(R) models do not have substantially deeper
potentials than expected in LCDM, then cosmological field amplitudes |f_R| >
10^{-6} will cause the galactic interior to evolve to low curvature during the
acceleration epoch. Viability of large-deviation models therefore rests on the
structure and evolution of the galactic halo, requiring cosmological
simulations of f(R) models, and not directly on solar-system tests. Even small
deviations that conservatively satisfy both galactic and solar-system
constraints can still be tested by future, percent-level measurements of the
linear power spectrum, while they remain undetectable to cosmological-distance
measures. Although we illustrate these effects in a specific class of models,
the requirements on f(R) are phrased in a nearly model-independent manner.
