A method based on image defocus information is disclosed for determining distance (or ranging) of objects from a camera system and autofocusing of camera systems. The method uses signal processing techniques. The present invention includes a camera characterized by a set of four camera parameters: position of the image detector inside the camera, focal length of the optical system in the camera, the size of the aperture of the camera, and the characteristics of the light filter in the camera. In the method of the present invention, at least two images of the object are recorded with different values for the set of camera parameters. The two images are converted to one-dimensional signals by summing them along a particular direction whereby the effect of noise is reduced and the amount of computations are significantly reduced. Fourier coefficients of the one-dimensional signals and a log-by-rho-squared transform are used to obtain a calculated table. A stored table is calculated using the log-by-rho-squared transform and the Modulation Transfer Function of the camera system. Based on the calculated table and the stored table, the distance of the desired object is determined. In autofocusing, the calculated table and the stored table are used to calculate a set of focus camera parameters. The camera system is then set to the focus camera parameters to accomplish autofocusing.