1. A process for the automatically determining a modulation transfer function of focal plane array cameras, said method comprising: providing a test grid, having an image with a spatial frequency in the detector plane that is detuned with respect to the Nyquist frequency or a rational fraction of the Nyquist frequency, of a detector array of said camera; using said test grid to generate a beat whose amplitude correspond to amplitude of the grid image to be measured, without the sampling the modulation transfer function; wherein the test grid covers a range of at least half a beat period; regardless of actual phase position, the modulation amplitude is determined therefore.