  We analyze the interference pattern produced by ultracold atoms released from
an optical lattice. Such interference patterns are commonly interpreted as the
momentum distributions of the trapped quantum gas. We show that for finite
time-of-flights the resulting density distribution can, however, be
significantly altered, similar to a near-field diffraction regime in optics. We
illustrate our findings with a simple model and realistic quantum Monte Carlo
simulations for bosonic atoms, and compare the latter to experiments.
