A point-symmetry function based on autoconvolution is described which makes it possible to track the position of point-symmetric objects with sub-pixel accuracy. The method is insensitive to grey level and was developed in order to have a fast and robust algorithm for real-time tracking of small magnetic particles in a light microscope. The phase contrast microscope image of the 4.5 micrometers diameter spherical particle consisted of concentric light and dark fringes where the shape of the fringes were dependent on the focus. THe position of the particle could be monitored in real-time at 25 Hz with a lateral accuracy of +/- 20 nm corresponding to less than +/- 0.1 pixel. To determine the vertical or z-position a new parameter was defined representing a measure of the second derivative of the intensity function. The vertical position could thus be determined with an accuracy of +/- 50 nm. The magnetic particle could be tracked and guided by applied magnetic fields to remain in a fixed position or programmed to scan either horizontal or vertical surfaces. Forces down to 10-14 N could by measured by monitoring the applied magnetic forces. One and two-dimensional Brownian motion could be studied by regulating the particle to a fixed z- position and monitoring the lateral position.