  We study the effect of modified gravity on weak lensing in a class of
scalar-tensor theory that includes $f(R)$ gravity as a special case. These
models are designed to satisfy local gravity constraints by having a large
scalar-field mass in a region of high curvature. Matter density perturbations
in these models are enhanced at small redshifts because of the presence of a
coupling $Q$ that characterizes the strength between dark energy and
non-relativistic matter. We compute a convergence power spectrum of weak
lensing numerically and show that the spectral index and the amplitude of the
spectrum in the linear regime can be significantly modified compared to the
$\Lambda$CDM model for large values of $|Q|$ of the order of unity. Thus weak
lensing provides a powerful tool to constrain such large coupling scalar-tensor
models including $f(R)$ gravity.
