A system for analysing a particle of interest of a biological sample comprising: - an analysis channel, comprising a first light source emitting an analysis light beam, a first optical system arranged to focus the analysis light beam in a plane orthogonal to the optical axis thereof, called focus plane (Π 0). and measurement means for measuring a characteristic of the analysis light beam having interacted with the particle of interest; - a position control channel, comprising a second light source, an image sensor, and a second optical system (150, 140) at least partially merged with the first optical system such that the first optical system and the second optical system comprise the same optical system, called shared optical system, the image sensor being offset relative to the image (Π I) from the focus plane by the second optical system (150, 140), along the optical axis of said second optical system (150, 140); and - calculation means characterised in that - the calculation means receive, as input, an image provided by the image sensor, and are adapted to calculate, from the image provided by the image sensor and by digital propagation, a series of images ( I 1 - I N) reconstructed into reconstruction planes each having an offset relative to the plane of the image sensor; - the calculation means comprise means for determining, from said series of reconstructed images, an offset relative to the plane of the image sensor, called optimum offset, said optimum offset being associated with the reconstruction plane which has a minimum offset with a plane receiving the image of the particle of interest by the second optical system, said plane receiving the image of the particle of interest being determined from the series of reconstructed images by determining a maximum or an inflection point of a function describing the evolution of a parameter of the reconstructed images depending on the offset of the reconstruction plane, said parameter being representative of the contrast of the reconstructed images; and - the calculation means comprise means of comparison between, on the one hand, the optimum offset and, on the other hand, the distance between the plane of the image sensor and the image (Π I) of the focus plane by the second optical system (140, 150), the result of the comparison determining an axial displacement setpoint value; and - the system comprises displacement means adapted to displace a support to receive the sample, in response to said axial displacement setpoint. - an analysis channel, comprising a first light source emitting an analysis light beam, a first optical system arranged to focus the analysis light beam in a plane orthogonal to the optical axis thereof, called focus plane (Π 0). and measurement means for measuring a characteristic of the analysis light beam having interacted with the particle of interest; - a position control channel, comprising a second light source, an image sensor, and a second optical system (150, 140) at least partially merged with the first optical system such that the first optical system and the second optical system comprise the same optical system, called shared optical system, the image sensor being offset relative to the image (Π I) from the focus plane by the second optical system (150, 140), along the optical axis of said second optical system (150, 140); and - calculation means - the calculation means receive, as input, an image provided by the image sensor, and are adapted to calculate, from the image provided by the image sensor and by digital propagation, a series of images ( I 1 - I N) reconstructed into reconstruction planes each having an offset relative to the plane of the image sensor; - the calculation means comprise means for determining, from said series of reconstructed images, an offset relative to the plane of the image sensor, called optimum offset, said optimum offset being associated with the reconstruction plane which has a minimum offset with a plane receiving the image of the particle of interest by the second optical system, said plane receiving the image of the particle of interest being determined from the series of reconstructed images by determining a maximum or an inflection point of a function describing the evolution of a parameter of the reconstructed images depending on the offset of the reconstruction plane, said parameter being representative of the contrast of the reconstructed images; and said optimum offset being associated with the reconstruction plane which has a minimum offset with a plane receiving the image of the particle of interest by the second optical system, said plane receiving the image of the particle of interest being determined from the series of reconstructed images by determining a maximum or an inflection point of a function describing the evolution of a parameter of the reconstructed images depending on the offset of the reconstruction plane, said parameter being representative of the contrast of the reconstructed images; and - the calculation means comprise means of comparison between, on the one hand, the optimum offset and, on the other hand, the distance between the plane of the image sensor and the image (Π I) of the focus plane by the second optical system (140, 150), the result of the comparison determining an axial displacement setpoint value; and - the system comprises displacement means adapted to displace a support to receive the sample, in response to said axial displacement setpoint.