Method for setting the laser focus of an excitation laser in a vessel through which blood flows, for the purposes of optical measurements for determining the sex of bird's eggs, wherein the vessels through which blood flows, including the embryo, are movable in the bird's egg and the vessels through which blood flows, including the embryo, are not yet fixated at the time of an optical measurement of the backscattered radiation at the shell, characterized by the following steps: - selecting a vessel through which blood flows in the hole-opened and incubated bird's egg by means of a contrast-based evaluation method while at least illuminating a vascular plexus through which blood flows with radiation in the green wavelength range by means of an optical apparatus for introducing laser radiation, for the purposes of detecting backscattered radiation and analysis position image areas SAF 1, SAF 2, - producing the laser focus for the excitation laser and a focus for a camera in the selected vessel through which blood flows, through the hole facing the vessel, by means of the optical apparatus for introducing laser radiation, for the purposes of detecting backscattered radiation and analysis position image areas SAF1, SAF2, - setting the laser focus within the selected vessel through which blood flows, by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2 by means of an erythrocyte movement-based difference image method on the basis of the intensity changes, occurring in respectively two successively recorded images on account of the movement of the erythrocytes within the selected vessel through which blood flows, as intensity values I t-1 (x), I t (x) at the pixel position x at times t-1 and t and tracking the laser focus and the focus of the camera when lowering the area of the blood vascular plexus and an albumen layer situated above the blood vascular plexus, - compensating occurring x-, y-horizontal movements of the selected vessel through which blood flows, by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2, - carrying out the optical measurements by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2. - selecting a vessel through which blood flows in the hole-opened and incubated bird's egg by means of a contrast-based evaluation method while at least illuminating a vascular plexus through which blood flows with radiation in the green wavelength range by means of an optical apparatus for introducing laser radiation, for the purposes of detecting backscattered radiation and analysis position image areas SAF 1, SAF 2, - producing the laser focus for the excitation laser and a focus for a camera in the selected vessel through which blood flows, through the hole facing the vessel, by means of the optical apparatus for introducing laser radiation, for the purposes of detecting backscattered radiation and analysis position image areas SAF1, SAF2, - setting the laser focus within the selected vessel through which blood flows, by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2 by means of an erythrocyte movement-based difference image method on the basis of the intensity changes, occurring in respectively two successively recorded images on account of the movement of the erythrocytes within the selected vessel through which blood flows, as intensity values I t-1 (x), I t (x) at the pixel position x at times t-1 and t and tracking the laser focus and the focus of the camera when lowering the area of the blood vascular plexus and an albumen layer situated above the blood vascular plexus, - compensating occurring x-, y-horizontal movements of the selected vessel through which blood flows, by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2, - carrying out the optical measurements by means of the optical apparatus for introducing laser radiation and for the purposes of capturing backscattered radiation and analysis position image areas SAF 1, SAF 2.