  Bandwidth smearing is a chromatic aberration due to the finite frequency
bandwidth. In long-baseline optical interferometry terms, it is when the
angular extension of the source is greater than the coherence length of the
interferogram. As a consequence, separated parts of the source will contribute
to fringe packets that are not fully overlapping; it is a transition from the
classical interferometric regime to a double or multiple fringe packet. While
studied in radio interferometry, there has been little work on the matter in
the optical, where observables are measured and derived in a different manner,
and are more strongly impacted by the turbulent atmosphere. We provide here the
formalism and a set of usable equations to model and correct for the impact of
smearing on the fringe contrast and phase, with the case of multiple stellar
systems in mind. The atmosphere is briefly modeled and discussed.
