In conventional X-ray imaging, the modulation transfer function (MTF) is the contrast-resolution relation. In a simplified X-ray imaging system, MTF is modeled by sinc functions of the detector size, the focal spot size, and any motion of the patient. Magnification improves the detector's MTF, but the focal spot MTF deteriorates. The optimal magnification is found by matching the first zeros of the MTFs for the detector and the focal spot.