  A massive foreground cluster lens changes the shapes (shear effect) and
number density (magnification effect) of the faint background galaxy
population. In this paper we investigate how the shear, magnification and
combined information can be used to constrain cluster mass profiles in the weak
lensing regime. We develop maximum likelihood techniques to make quantitative
predictions for each of the methods. Our analytic results are checked against
Monte Carlo simulations. In general, we find that the shear method is superior
to the magnification method. However, the magnification information complements
the shear information if the former has accurate external calibration. For the
magnification method, we discuss the effects of random and systematic
uncertainties in the background galaxy counts.
