Signal-to-noise ratios are derived for an interferometric array of N diffraction-limited optical/IR telescopes with a Fizeau-type beam recombination. We consider two types of objects—extended sources and point sources—and three sources of noise—signal photon noise, sky background noise, and detector noise. We compare the results with that of a filled aperture, and also to that of an incoherent array, each with the same total collecting area. Implications are briefly discussed.