The specific calculation required for the analytical determination of the OTF depends on a number of factors. First, the choice depends on whether the calculation is (1) a purely theoretical one for a hypothetical optical system having a given kind and amount of aberration, (2) an anticipated evaluation from system design data for optics in the design stage, or (3) a practical determination from experimental results for an actual system that is being tested. In the calculation from experimental results, the method depends on the information available, for example, whether the pupil has been measured interferometrically or the aerial image of a point source (the aerial spread function) has been scanned by one of several methods: by a knife edge, an elongated slit, or a "point" sensor.