When waves propagate through a medium with smooth velocity perturbations the propagation of wavefronts is determined by the eikonal equation. Here the propagation of wavefronts through a medium with small-scale velocity perturbations is analyzed using the method of strained coordinates. A partial differential equation for the first-order perturbation of wavefronts is derived that includes frequency-dependent wave propagation effects. It is shown that for the special case of the scalar Helmholtz equation with a homogeneous reference medium this leads to the same perturbation of wavefronts as obtained from the Rytov approximation. However, the method presented here can possibly be generalized to more complex wave propagation problems where the Rytov approximation cannot be used, such as the propagation of vector waves.