An apparatus for determining ametropia of an eye includes an optical assembly with a light source, a detector, a plurality of optical elements and at least one stray light stop and a controller. An illumination beam path is provided between the light source and an optical interface in order to allow illumination light generated by the light source to emerge from the optical interface. A measurement beam path is provided between the optical interface and the detector in order to supply measurement light entering through the optical interface to the detector. The measurement beam path passes through an aperture of the at least one stray light stop. A diameter of this aperture is variable or a position of this aperture along the measurement beam path is variable in order to reduce stray light at the detector.
