We study a local feature of a Newton logarithmic barrier function method and a Newton primal-dual interior-point method. In particular, we study the radius of the sphere of convergence of Newton's method on two equivalent systems associated with the two aforementioned interior-point methods for nondegenerate problems in inequality contrained optimization problems. Our theoretical and numerical results are clearly in favor of using Newton primal-dual methods for solving the optimization problem. This work is an extension of the authors' earlier work [10] on linear programming problems.