Two-point correlation functions of spin operators in the minimal models
${{\cal M}}_{p,p'}$ perturbed by the field $\Phi_{13}$ are studied in the
framework of conformal perturbation theory. The first-order corrections for the
structure functions are derived analytically in terms of gamma functions.
Together with the exact vacuum expectation values of local operators, this
gives the short-distance expansion of the correlation functions. The
long-distance behaviors of these correlation functions in the case ${{\cal
M}}_{2,2n+1}$ have been worked out using a form-factor bootstrap approach.
The results of numerical calculations demonstrate that the short- and
long-distance expansions match at the intermediate distances. Including the
descendent operators in the OPE drastically improves the convergency region.
The combination of the two methods thus describes the correlation functions at
all length scales with good precision.