  Analysis of the imaging of some simple distributions of object phase by a
phase plate of Zernike type shows that sharp transitions in the object phase
are well transmitted. The low-frequency components of the complete object
function are attenuated by the plate. The behaviour can be characterised by a
cut-on parameter defined as the product of the cut-on frequency of the plate
and a characteristic dimension of the object. When this parameter exceeds a
value of the order of unity, a sharp boundary in the object is imaged by a
Zernike plate as a dark lining inside the boundary with a white outline or halo
outside the boundary, in agreement with reported observations. The maximum
diameter of objects that can be imaged accurately is inversely proportional to
the diameter of the hole for beam transmission in the phase plate.
