The double-pass method is applied to determine the optical image quality in monofocal contact lens (CL) wearers. This is an objective non-invasive technique that permits in vivo testing of the optical performance of CL wearers' eyes. Retinal image quality was measured for three subjects wearing two types of monofocal CLs: a rigid gas permeable (RGP) CL and a soft contact lens (SL), for pupil diameters of 3 mm and 5 mm. Results show the importance of ocular astigmatism regarding retinal image quality. In eyes presenting corneal astigmatism, the best results are obtained when wearing RGP CLs, because the lens compensates the corneal astigmatism. The modulation transfer function (MTF) is considerably smaller when no lens or soft lenses are worn, even for small amounts of astigmatism (0.5 D). When the astigmatism is corrected, the retinal image quality obtained with both types of CLs and with no lens is similar