The Rasnik system is a 3-point optical displacement monitor with sub-nanometer precision. The CCD-Rasnik alignment system was developed in 1993 for monitoring the alignment of the muon chambers of the ATLAS Muon Spectrometer at CERN. Since then, the development has continued as new CMOS imaging pixel chips became available. In this work the system processes and parameters that limit the precision are studied. We conclude that the spatial resolution of Rasnik is only limited by the quantum fluctuations of the photon flux arriving at the pixels of the image sensor. The results of two Rasnik systems are compared to results from simulations, which are in good agreement. The best spatial resolution obtained was 7 pm/Hz. Finally, some applications of high-precision Rasnik systems are set out.