  The out-of-plane vibration of a rough surface causes an in-plane vibration of
its speckle pattern when observed with a defocused optical photographic system.
If the frequency of the oscillations is high enough, a time-averaged
specklegram is recorded from which the amplitude of the vibration can be
estimated. The statistical character of speckle distributions along with the
pixel sampling and intensity analog-to-digital conversion inherent to
electronic cameras degrade the accuracy of the amplitude measurement to an
extent that is analyzed and experimentally tested in this paper. The relations
limiting the mutually competing metrological features of a defocused speckle
system are also deduced mathematically.
