1. A method for determining flow conditions in a measuring volume (VR), which permeated by a fluid spiked with optically detectable particles, comprising the following steps: a) simultaneously recording at a first recording point in time (t 0 ) of a first plurality of two-dimensional, real images (R 10 , R 20 ) of a three-dimensional, real distribution of the particles by means of the same plurality of image detectors ( 12 , 14 ) arranged spatially offset with respect to one another, b) repeating step a) at least at one second recording point in time (t 1 ), c) determining for each recording point in time (t 0 , t 1 ) a three-dimensional, estimated distribution (VI 0 , VI 1 ) of the particles based on the real images and d) calculating a three-dimensional displacement vector field (VV) by comparing the estimated distributions (VI 0 , VI 1 ), wherein e) determining for each image detector ( 12 , 14 ) an optical transfer function (F 1 ; F 2 ) with which the real distribution (VR) is imaged by the image detector ( 12 ; 14 ) before carrying out step c), and wherein step c) starts with a roughly estimated starting distribution (VIstart) as an estimated distribution (VIj) and thereafter is carried out iteratively with the following sub-steps: c1) calculating two-dimensional, virtual images (I 1 j , I 2 j ) of the estimated distribution on the basis of the transfer functions (F 1 , F 2 ) determined in Step e, c2) comparatively determining differences between the virtual images (I 1 j ; I 2 j ) and the respectively assigned real images (R 1 j ; R 2 j ), c3) changing the estimated distribution (VIj) depending on the determined differences, and wherein the iteration is ended when a predetermined termination criterion has been reached.