The shape of an internal magnetic shield in a TV tube determines in a complex way to what extent the influence of the earth's magnetic field on the TV image is reduced. Conformal mapping and finite element calculations of the field distribution in models of shields were performed to clarify these effects. In a two-dimensional model of a shield, the mislanding that occurs outside the shield can be more than compensated for by the field distribution inside the shield. In an axially symmetric three-dimensional model, compensation is weaker, because the fields inside the shield are lower.