Experiments are described in which monocular accommodation response/stimulus curves were measured with Snellen targets over the stimulus range from +1.0 to -5.0 D, using artificial pupils with diameters of 1.0, 1.5, 2.0, 2.5 and 3.0 mm and a constant retinal illuminance of 600 td. The results agree with those of earlier authors in showing a diminished response with smaller pupils. The slopes of the quasi-linear central portions of the response/stimulus curves are well described in terms of a geometrical optical approximation in which the accommodation system works to produce a retinal blur circle whose diameter is a linear function of the dioptric difference between the magnitudes of the stimulus and the accommodative resting state, this blur circle diameter being independent of the pupil diameter. Further consideration of diffractive effects suggests that contrast changes in the intermediate spatial frequency components (approximately 5 c/deg) of the retinal image may play a dominant role in guiding the response.