However, similar to a real engine piston moving up and down in its cylinder, optical piston values can be changed to bias the wavefront phase mean value as desired.
This property is critical to the operation of phase-measuring interferometers, which give not only the magnitude but also the sign (convexity or concavity) of a wavefront under test.
Piston is physically created in the interferometer by piezoelectric actuators that translate the Fizeau interferometer reference surface along the optical axis by precise fractions of the test wavelength, usually by one quarter of a wavelength.
This changes the interferometric fringe patterns and allows direct calculation of the exact wavefront error.
Piston and tilt are not actually true optical aberrations, as they do not represent or model curvature in the wavefront.