A method of analyzing a sample receiving a particle of interest, including: defining a reference point located on a first interface of the sample, or at a known distance from the sample, along the optical axis of the optical system; acquiring a reference image transmission of the sample, the object plane of the optical system being located at a known distance from the reference point along an axis parallel to the optical axis of the optical system, and the particle of interest being located outside of the object plane; using the reference image, digitally constructing a series of reconstructed images, each associated with a predetermined offset of the object plane along the optical axis of the optical system; and using the series of reconstructed images, determining the distance along an axis parallel to the optical axis of the optical system, between the particle of interest and the reference point.