A method of autofocusing a Gaussian beam with obscuration onto a small target, down range and on the beam axis, is presented. For a source with a sinusoidally time varying focus (dither) the temporal spectra of the reflected signal can be very accurately calculated for ranges where Fresnel approximations are valid. For such ranges certain spectral components in the reflected signal undergo significant phase and amplitude changes while passing through true focus. The closed loop control strategy naturally evolves from these properties.