Studies of the relationship between luminance and pupil diameter have produced widely differing results. This research note explores the possibility that this is due, in part, to differences in the size of the adapting fields used by various workers. We present measurements of pupil diameter as a function of luminance for a variety of field subtenses. The results indicate a consistent trend for smaller subtenses to produce less pupil constriction. For field diameters of up to 25°, replotting the data in terms of corneal flux density (i.e. the product of luminance and subtended area) causes an approximate convergence onto a single function described by D = 7.75 − 5.75 [(F/846)0.41/((F/8460.41 + 2)] where D is the pupil diameter (mm) and F is the corneal flux density (cd m−2 deg2). This equation should be of some practical use in estimation of natural pupil diameter.