The description and use of a non-perturbative optical diagnostic technique for measuring sheath movement in plasma focus are presented. The method is based on a lens that collects the light emitted by the plasma sheath which is detected by fast photodiodes. Two different circuit connections for the photodiodes are presented using either several or a single scope channels to record the photodiode signals. The technique is applied to a 300 J plasma focus with a spatial resolution of ∼0.25 mm measuring its average propagation speeds (axial and radial) as well as its detachment time and an estimated thickness. The setup required in this technique is easy to implement and cost-effective, and it can be applied in any plasma focus, regardless of its stored energy or operational gas.