We attempt to measure image quality to determine the performance of an optical system so that its range of applicability and the information content of the data collected can be specified. Why is there any problem? Mainly because we always attempt to use as simple a measure as possible to describe what is in fact a fairly complicated process. Furthermore as various measures have been used (and abused!) the technology of system design has always been more sophisticated than the measure was designed for. A simple example of this is the two-point resolution criterion first propounded by Rayleigh. The criterion which only truly applies to the resolution of two incoherent individually unresolved objects is directly applicable to the specification of performance of a telescope in viewing star fields; it is also applicable (under carefully specified and controlled conditions) to a microscope viewing fields of individual cells. However, it is not of much value in specifying performance of a telescope viewing the moon, mars or other planets; or to a microscope viewing a general biological specimen.