  Gravitational light deflection can distort the images of distant sources by
its tidal effects. The population of faint blue galaxies is at sufficiently
high redshift so that their images are distorted near foreground clusters, with
giant luminous arcs being the most spectacular evidence for this effect. Much
weaker distortions, however, can observationally be detected by a statistical
analysis of the numerous faint galaxy images, as first demonstrated by Tyson,
Valdes \& Wenk. This distortion effect can be used as a {\it quantitativetool}
for the reconstruction of the surface mass density of galaxy clusters with
appropriate redshifts, as was demonstrated by Kaiser \&Squires. They have
derived an explicit equation for this surface mass density in terms of its
tidal field. The reconstruction formula by Kaiser \& Squires must be modified
because of two effects: in its original form it applies only to weak lenses,
and hence must be generalized to account for stronger lensing effects. Second,
due to the nature of the inversion formula, it produces boundary artefacts (or
biases) if applied to real data which are confined to a finite field on the
sky. We discuss several possibilities to obtain inversion formulae which are
exact for ideal data on a finite data field (CCD). We demonstrate that there
exists an infinite number of such finite-field inversion formulae, which differ
in their sensitivity to observational effects (such as noise, intrinsic
ellipticities of the sources, etc.). We show that, using two simple conditions,
one can uniquely specify a finite-field formula which in a well-defined sense
minimizes the sensitivity to observational effects. We then use synthetic data
to compare the quality of our new reconstruction method with that of previous
