1. A device for obtaining a vital sign of a subject, comprising: an interface that receives a set of image frames of a subject, an analysis unit that determines the amount of direction changes and/or the time distances between direction changes within a region of interest in a subset of image frames comprising a number of image frames of said set, a direction change indicating a change of the direction of motion appearing within said region of interest, an evaluation unit that determines if said region of interest within said subset of image frames comprises a vital sign information and/or noise by use of the determined amount of direction changes and/or the time distances for said subset of image frames, and a processor that determines the desired vital sign of the subject from said region of interest within said subset of image frames if it is determined that said region of interest within said subset of image frames comprises a vital sign information.