Keratometer is also known as ophthalmometer. It is a diagnostic instrument used by ophthalmologists for examining eye. It helps in measuring the curvature of the anterior surface of the cornea specially for assessing the extent of astigmatism (it is a refractive error in which eye does not focus light on retina and the object looks blur). Apart from that, it provides an accurate measurement of corneal radius curvature and curvature of contact lens. Its principle is based on distance of the object and image distance ratio, plus the reflective surface, and the object radius with the reflective surface. With this one can calculate the deviation of the light from the retina.