1. An ophthalmic instrument comprising: an extended source producing light that is formed as an image on the retina of the human eye and reflected there from to produce retinal reflections derived from said extended source; and a wavefront sensor that estimates aberrations in said retinal reflections, wherein said wavefront sensor comprises: a plurality of subapertures that form a plurality of images of the extended source from said retinal reflections; an imaging device that captures said plurality of images and outputs image data representing said images; and an image processing computer that generates an estimate of the gradient field of said retinal reflections by applying image correlation techniques in the digital domain to said image data.