  We present a field-theoretic study of the critical Casimir force of the Ising
universality class in a $d$-dimensional ${L_\parallel^{d-1} \times L}$ slab
geometry with a finite aspect ratio $\rho = L/L_\parallel$ above, at, and below
$T_c$. The result of a perturbation approach at fixed dimension $d=3$ is
presented that describes the dependence on the aspect ratio in the range $\rho
\gtrsim 1/4$. Our analytic result for the Casimir force scaling function for
$\rho = 1/4$ agrees well with recent Monte Carlo data for the three-dimensional
Ising model in slab geometry with periodic boundary conditions above, at, and
below $T_c$.
