A diagnostic device detects a layer boundary which facilitates the determination of layer thicknesses in the human eye. The device has a light source with a beam path which is designed to guide at least one measuring beam of the light source from the object plane of the light source into an intersecting region of the measuring beam with an optical axis in the eye, and an actuator designed to move the intersecting region along the optical axis. The beam path guides the measuring beam from a layer boundary into the sensor unit. An evaluation unit is designed to estimate and/or determine a layer thickness between a first and a second layer boundary.