An optical interference apparatus for carrying out Fourier domain optical coherence tomography. Multiple beams are provided and respective interferograms are recorded simultaneously for a plurality of different focal depths within a substance to be examined. Combined images are derived from the interferograms for a plurality of different focal depths, whereby a single image may be constructed with an increased depth of field. The axial spacing of the foci is calculated to take into account the Rayleigh range of the focal waist in the substance to be examined.