Apparatuses and methods for determining a refractive error of an eye are disclosed. A series of images of light coming from an eye are captured with varying optical powers, and the refractive error is then calculated based directly on the series of images used as approximate point spread functions. The calculation includes determining a modulation transfer area as a function of meridian angle and optical power in an angle range from 0° to 180° based on the series of images, and to calculate the refractive error based on the modulation transfer area as a function of angle and optical power.