It is important to characterize and quantify the imaging performance of optical coherence tomography (OCT) and especially optical coherence microscopy (OCM) systems for validation and calibration of the setup. We suggest using a simple rubber surface, presented to the setup at different focal positions, to measure the system’s lateral resolution and depth of field. Rubber is a nearly perfect scatterer and produces a fully developed speckle pattern. The speckle size and shape are directly defined by the optical system and its illumination and detection apertures. Analysis of the speckle pattern reveals the system’s coherent transfer function (CTF) and hence the point spread function (PSF) in a straightforward computation.