The change in pupil centration occurring as the human eye pupil dilates, both naturally in darkness and following the use of a mydriatic drug (cyclopentolate HCl, 1%), has been measured photographically. In both cases, changes in centration of up to 0.4 mm have been found. Using available ocular wavefront aberration data, modulation transfer curves have been derived for two eyes for centration changes of 0.2 mm. The importance of these changes in the formation of the retinal image, and hence on visual performance, is discussed.