AbstractVideo and digital image processing have been used to amplify the contrast of light microscopic images, making it possible to observe in real time the diffraction images of cell structures 10 times smaller than the Raleigh resolution limit of 0.2 μm. In this paper we discuss how quantitative analysis of diffraction images can be used to extract information about motion or structure at the nanometer level. This issue is considered in the context of a method for tracking the motion of kinesin‐coated beads on microtubules with 1–2 nm precision (Gelles et al.:Nature331:450–453,