A keratometer is an instrument that measures the curvature of the cornea to detect and quantify astigmatism. It provides measurements of the flattest and steepest meridians of the cornea in diopters. Manual keratometry is preferred for certain patients and when performing toric IOL calculations, while corneal topography can provide additional information about the anterior corneal surface. Proper use and calibration of the keratometer is important for obtaining accurate measurements.