A manually-operated ophthalmic device intended to be placed in front of a patient's eyes to assist with measuring the refractive error of the eye (the inaccuracies of focusing light upon the retina) during an examination, typically to determine a patient's prescription for glasses. Also known as a manual refractor, it typically consists of two mounted disk-like housings (head) which contain lenses of different characteristics. The optometrist manually changes the lenses and other settings while asking the patient to read an eye chart and provide subjective feedback on which settings give best vision.