A source of light having many times the angular subtense of the sun or moon can usually be treated as a point when making photometric calculations based on the inverse square law. When viewed by the eye, such sources are not perceived as points (like stars) but rather as extended surfaces. In this paper, some typical data on contrast thresholds for targets, both large and small, are converted into threshold stellar magnitudes. This is done for two important cases which, unless carefully distinguished, can lead to erroneous results.
ARTHUR C. HARDY, "How Large is a Point Source?," J. Opt. Soc. Am. 57, 44-47 (1967)