The nonhomogeneity of a sample in the positron implantation range affects the measured positron annihilation characteristics. The calculation and computer simulations of this situation were performed. They showed that positrons emitted from a point source isotropically penetrate a layer of thickness equal to three times reciprocal of the linear-absorption coefficient that is 2 times smaller than in the case of positrons formed in a beam. The presented formulas and results are useful for analysis of the positron annihilation experiments applied to the study of the defect distribution profile.