The Prentice position is an orientation of a prism, used in optics, optometry and ophthalmology. In this position, named after the optician , the prism is oriented such that light enters it at an angle of 90° to the first surface, so that the beam does not refract at that surface. All the deviation caused by the prism takes place at the exit surface. In ophthalmology, were classically calibrated for use in the Prentice position, while were calibrated for use in the frontal position.