  We find exact differential mean numbers of fermions and bosons created from
the vacuum due to a composite electric field of special configuration. This
configuration imitates a finite switching-on and -off regime and consists of
fields that switch-on exponentially from the infinitely remote past, remains
constant during a certain interval $T$ and switch-off exponentially to the
infinitely remote future. We show that calculations in the slowly varying field
approximation are completely predictable in the framework of a locally constant
field approximation. Beyond the slowly varying field approximation, we study
effects of fast switching-on and -off in a number of cases when the size of the
dimensionless parameter $\sqrt{eE}T$ is either close or exceeds the threshold
value that determines the transition from a regime sensitive to on-off
parameters to the slowly varying regime for which these effects are secondary.
