Schematic representation of the effect of the finite size of reciprocal-lattice points on the diffraction images. The black ellipse represents a reciprocal-lattice point of finite size that results from the combination of crystal mosaicity, beam divergence, wavelength dispersion and variability in unit-cell parameters. The arcs represent the positions of the Ewald sphere at the beginning and end of one or more images. When the oscillation angle is large compared with the angular width of the reciprocal-lattice point (left), the entire reciprocal-lattice point lies between the two arcs and all the intensity is recorded on a single image (a fully recorded reflection). If the oscillation angle per image is halved (centre), the total intensity is distributed over two images (partially recorded reflections). If the oscillation angle is significantly less than the reflecting range, the intensity is distributed over several images (right), resulting in fine φ-slicing. Figure after Elspeth Garman.