The rhabdoms of the eyes of butterflies can support one, two, or more waveguide modes, depending on their diameter. The pupil mechanism consists of pigment granules that migrate towards the rhabdom in the light, and absorb the modal light that travels outside the rhabdom. Because there is relatively more extra-rhabdomal light in the higher-order modes, these are shed earlier and at lower light intensities, than the first mode. The effect of this is to reduce the acceptance angle of each ommatidium. We have measured the extent and timecourse of this change by both optical and electrophysiological methods. In small butterflies whose rhabdoms support only one mode there is no change. In Nymphalids such as Vanessa itea, where there are two modes, the reduction is by ca. 30% and in the crepuscular Satyrid Melanitis leda where three or more modes are present the reduction is almost 50%, from 2.9 degrees to 1.5 degrees. These pupil-induced acuity changes will occur towards the lower end of the range of environmental luminances seen by the butterfly during natural flight.