Resolution and/or coherence length is a property of a diffraction instrument and is independent of the characteristics of the sample. The so-called resolution "ellipsoids" for reflectometers are shown in a graphical way, which should help in planning and optimizing the data collection for an experiment. Mathematical expressions for exact calculations of the ellipsoids are derived for three orthogonal directions, with the component of resolution perpendicular to the scattering plane given explicitly. All the resolution components are given in terms of the instrument parameters: wavevector; angles; and their variations. Particular attention is paid to the investigation by reflectometry of gratings, optimizing for specular and off-specular measurements, and, in the case of the grating, being rotated such that its stripes are not perpendicular to the incoming beam.