Ophthalmic imaging systems, particularly slit-scanning ophthalmo-scopes, are capable of characterizing refraction over the entire field of view of the system. Light from the light source of the system illuminates a region of the eye and the returning light is measured on a detector. The deviation of the location of the returning light from a predetermined location on the detector is measured. The deviation corresponds to the mismatch between the refractions of the imaging system and the eye. The light can be scanned across the full field of view to characterize the entire field. A second illumination source traveling along a second illumination path can be used to improve the characterization. The characterization can be of use for optimizing the focus of the instrument and for assessing the condition of the eye, including assessing myopia and astigmatism in the periphery.