The aim of the present invention is to propose a diagnostic device for detecting a layer boundary which facilitates the determination of layer thicknesses in the human eye. For this purpose, a diagnostic device 1 for detecting a layer boundary in an eye is proposed, with a light source 7 7a, b, wherein the light source defines an object plane, with a sensor unit 17, 17a, 31, 32, with a beam path which is designed to guide at least one measuring beam 13 of the light source 7 7a, b from the object plane of the light source into an image plane and/or into an intersecting region of the measuring beam 13 with an optical axis 15 in the eye 2, with an actuator 12, 30 designed to move the image plane and/or the intersecting region along the optical axis 15, wherein the beam path is designed such that, in a detection state, the measuring beam 13 is guided from a layer boundary into the sensor unit 17, 17a, 31, 32 if the image plane and/or the intersecting region is located on the layer boundary, and with an evaluation unit 18 designed to recognize the detection state on the basis of the signals of the sensor unit 17, 17a, 31, 32, wherein the evaluation unit 18 is designed to estimate and/or determine a layer thickness d1, d2, d3 between a first and a second layer boundary on the basis of the position of the actuator 12, 30 in a detection state of the first and the second layer boundary.
