Self-focusing by a Kerr nonlinearity in combination with an intracavity aperture creates a power-dependent loss in lasers, which has been used as a method for passive mode locking of lasers. An analytical treatment is presented that yields closed-form expressions for the modulation efficiency. Comparisons between analytical results and numerical calculations are performed. General features of this mode-locking technique are discussed, and a procedure for the optimization of hard-aperture Kerr-lens mode-locking performance is given.