Purpose: For the focal spot measurement of x-ray tubes, we propose a practical method in which only a metal edge and a digital detector are used, together with a process of removing detector blur inherently associated. Approach: The evaluation was made through the optical transfer function (OTF) measurements using the edge response of a 1-mm-thick tungsten plate. First, we made the acquisition of a geometrically magnified edge response, which consists of focal spot penumbra and detector blur, followed by the acquisition of nonmagnified edge response, which includes only detector blur. Then the detector blur was removed by taking the ratio of the two OTFs. Finally, the focal spot profile was obtained by the inverse Fourier transform of this ratio. Results: Resultant full widths at the half-maximum of a small focus profile were for the proposed method and for the conventional slit method with film, indicating excellent agreement between both methods. Comparing between results obtained using two flat panel detectors with different pixel pitches (0.143 and 0.175 mm) confirmed no differences with these variations. Conclusion: Through the whole study, the accuracy and the practicality of the proposed method were demonstrated, indicating a possibility of the method to be widely used to evaluate the effective focal spot size and profile of x-ray systems.