We study the magnetization in(L)(h, beta) for the Ising model on a large bu
t finite lattice square under the minus boundary conditions. Using known la
rge-deviation results evaluating the balance between the competing effects
of the minus boundary conditions and the external magnetic field h, we desc
ribe the details of its dependence on h as exemplified by the finite-size r
ounding of the infinite-volume magnetization discontinuity and its shift wi
th respect to the infinite-volume transition point.