AMBER is the near I.R. focal instrument of the VLTI. It recombines 3
beams simultaneously and has spectral resolution capability allowing
simultaneous spectrum, visibilities, differential visibilities,
closure phase and differential phases measurements on the observed
sources. We present here the general formalism used on AMBER in
order to compute differential phases and differential visibilities,
in complement to the article of Tatulli et al. (2006). We show that
these differential observables have precisions close to the
fundamental noises and we present an illustration of
model-fitting using them on the binary star γ2 Velorum where we
made a precise measurment of the astrometry of the 2 components
(3.65±0.1 mas).