ISO 20462-1:2005 is part of a three-part standard pertaining to the subjective evaluation of pictorial still image quality. ISO 20462-1:2005 defines the units by which image quality is quantified (just noticeable differences, or JNDs); describes the influence of stimulus properties, observer characteristics, and task instructions on results obtained from rating experiments; and provides a flow chart for choosing the preferred psychophysical method for determining image quality from among those defined in subsequent parts of ISO 20462.