  Non-local thermodynamical equilibrium (NLTE) line formation for neutral and
singly-ionized calcium is considered through a range of spectral types when the
Ca abundance varies from the solar value down to [Ca/H] = -5. Departures from
LTE significantly affect the profiles of Ca I lines over the whole range of
stellar parameters considered. However, at [Ca/H] >= -2, NLTE abundance
correction of individual lines may be small in absolute value due to the
different influence of NLTE effects on line wings and the line core. At lower
Ca abundances, NLTE leads to systematically depleted total absorption in the
line and positive abundance corrections, exceeding +0.5 dex for Ca I 4226 at
[Ca/H] = -4.9. In contrast, NLTE effects strengthen the Ca II lines and lead to
negative abundance corrections. NLTE corrections are small, <= 0.02 dex, for
the Ca II resonance lines. For the IR lines of multiplet 3d - 4p, they grow in
absolute value with decreasing Ca abundance exceeding 0.4 dex in metal-poor
stars with [Fe/H] <= -3. Ca abundances are determined for the Sun, Procyon, and
seven metal-poor stars, using high S/N and high-resolution spectra at visual
and near-IR wavelengths. Lines of Ca I and Ca II give consistent abundances for
all objects (except Procyon) when collisions with hydrogen atoms are taken into
account. The derived absolute solar Ca abundance (from Ca I and Ca II lines) is
$\eps{Ca,\odot}$ = 6.38+-0.01. For Procyon, the mean Ca abundance from Ca I
lines is markedly subsolar, [Ca/H] = -0.14+-0.03. The W(Ca I 4226)/W(Ca II
8498) equivalent width ratio is predicted to be sensitive to surface gravity
for extremely metal-poor stars, while this is not the case for the ratio
involving the Ca II resonance line(s).
