We use the paraxial geometric optics model of image formation to derive a set of camera focusing techniques. These techniques do not require calibration of cameras but involve a search of the camera parameter space. The techniques are proved to be theoretically sound. They include energy maximization of unfiltered, low-pass filtered, high-pass filtered, and band-pass filtered images. It is shown that in the presence of high spatial frequencies, noise, and aliasing, focusing techniques based on band-pass filters perform well. The focusing techniques are implemented on a prototype camera system named SPARCS. The architecture of SPARCS is described briefly. The performance of the different techniques are compared experimentally. All techniques are found to perform well. One of them -- the energy of low pass filtered image gradient -- which has better overall characteristics is recommended for practical applications.