The advantage of film methods is that large numbers of relections can be collected at the same time, on one piece of film, but a large amount of human intervention is necessary to initially align the crystal and to change and develop films; the four-circle diffractometer collects only one reflection at a time, but it runs unattended until data collection is completed. Orienting and measuring the intensity of a single reflection takes approximately 30-60 seconds; so for a small molecule data set of, for example, 4000 relections, data collection would require 2-3 days of 24 hour operation to collect a complete set of reflections. Once the unit cell has been determined, the diffractometer software generates a list of reflections to collect and data collection proceeds. An algorithm will be supplied to the software that establishes which reflections need to be collected to ensure a complete, unique data set. For example, in the monoclinic crystal class there are two sets of symmetry-related reflections:

One of the sign combinations in each set must be collected. For example, if reflections are collected according to the algorithm:


the classes will be collected (h_max, k_max, and l_max are the maximum h, k, and l indices as determined by a maximum 2θ angle - often 50° or 55° for molybdenum radiation.