Beam mixing with a pinhole

After a small aperture the spatial information of a complex optical wavefront is lost, but amplitude and phase information is mixed and transferred to the smoothed wave that emerges from the pinhole. This mixing effect is described in the case of a wavefront with a phase step, which is shifted over the input plane of an optical processor with a pinhole as spatial filter in the Fourier plane. We constructed a polarizing interferometer to demonstrate this continuous phase shift and show that it can be used as a variable retardation wave plate similar to a birefringent compensator, but without crystalline wedges.