in shape fig1 represents an irradiation of the object 3 in a plurality in the present case two different positions 1 2 in the first position 1 in this example the object 3 is irradiated parallel to the short semi axis of the ellipse formed by the object 3 in the cross section in the second position 2 perpendicular to the foregoing and parallel to the long semi axis accordingly in this case a higher individual dose which is deposited in the object 3 for example a patient 39 s body is to be expected in the second position 2 than in the first position 1 for an identical image quality q fig2 this may be explained by the fact that if a homogeneous composition of the object 3 is assumed a greater extent of an object in one direction goes hand in hand with a greater absorption of an x ray that is passing through the object than is the case for a smaller extent along a direction of irradiation with respect to an image quality q fig2 of a three dimensional reconstruction which is dependent on image signals captured in the present two positions 1 2 the quality of the three dimensional reconstruction remains unchanged on the basis of the error propagation if in the case of a decrease in the image quality.