The model for post-refinement in MOSFLM. The large circle represents a cross-section of the Ewald sphere. A reciprocal-lattice point is shown as a shaded circle centred on P′, representing its position at the end of the first of two images on which this reflection is recorded. The reciprocal-lattice point rotates clockwise during data collection, so the intensity recorded on the first image will be proportional to that part of the shaded circle that lies outside the Ewald sphere. P represents the calculated position of this reciprocal-lattice point using the current crystal parameters. The angle subtended by the points P, P′ at the origin of the reciprocal lattice is minimized during post-refinement of the crystal parameters.