In a process for automatically determining the modulation transfer function (MTF) of a focal plane array (FPA) cameras, a test grid is used to generate a beat whose amplitude corresponds to that of the grid image to be measured, without the sampling MTF. (The image of the test grid has a spatial frequency in the detector plane that is detuned with respect to the Nyquist frequency of the detector array. The test grid covers a range of at least half a beat period, and then, regardless of the actual phase position, the modulation amplitude is determined therefrom. Comparative MTF measurements are thereby permitted without the influence of the sampling effect of an FPA, and particularly the MTF measurement in real time suitable for the application to moving images.