an x - ray apparatus is equipped with an x - ray filter having adjustable absorption properties in order to reduce the dynamic range of an x - ray image . the filter includes a matrix of filter elements 13 , 13 ′, 13 ″ in which a level of an x - ray absorption liquid can be varied . the amount of the x - ray absorption liquid within an individual filter element determines the value of the x - ray absorption within this filter element . it is necessary that the resultant amount of the x - ray absorption liquid within each filter element is reproducible . such an x - ray filter is mounted near the x - ray source and is rotated together with the gantry when an x - ray image is required with some angulation with respect to an object . when the x - ray filter is angled with respect to the vertical direction , the hydrostatic pressure within filter elements varies with respect to the calibrated state and the resultant amount of the x - ray absorption liquid is not reproducible . in order to compensate the variation of the hydrostatic pressure within the filter , the latter is provided with a hydrostatic pressure control system .