Pupil fatigue: effectiveness of pupil-dilating drops

Participants will have pupil diameters measured using an infrared video pupillometer, and then have one drop of either a) tropicamide 1% alone, b) tropicamide 1% and phenylephrine 2.5% a few seconds apart, or c) cyclopentolate 1% and phenylephrine 2.5% a few seconds apart, instilled into one eye. After an appropriate delay to allow action of the mydriatic agent (30 minutes except for cyclopentolate = 60 minutes), pupillometry will be performed again. All participants except group c) (the 'control' group) will return the following day to have pupillometry performed again, then one drop each of cyclopentolate 1% and phenylephrine 2.5% will be instilled, and pupillometry repeated after 60 minutes.