Using an ultrafast optical technique we image near-gigahertz flexural acoustic waves propagating in a thin gold wedge deposited on an ultrathin silicon nitride slab. Temporal Fourier transforms of the imaged data in two dimensions allow individual frequencies to be accessed. The wave fronts associated with antisymmetric Lamb waves bend towards the thin end of the wedge. This behaviour is mimicked with an analytical model based on sections of a linear wedge. We also conduct numerical simulations which show good agreement with the experimental results.