In field emission x‐ray tubes, the target is conical in shape and the projection of the focal spot along the central axis a circular disk. To determine the resolution capability of such tubes, a theoretical expression has been derived for the MTF of a circular focal spot. It has been shown that the MTF is given by the equationMTF = 2J1[πf0δ(m − 1) / m] / [πf0δ(m − 1) / m]wheremis the magnification factor,f0the spatial frequency in the object plane, and δ the diameter of the focal spot.J1stands for the Bessel function of the first order. The above expression has been tested experimentally and used to compare circular focal spots with square focal spots.