Formulae are derived which calculate the bounds of discrete array MTF caused by the non-stationary nature of such devices. It is shown that if the traditional sinc based description of discrete device MTF is used, overestimation occurs. The average of the derived bounds is shown to be a good estimate of the SFR of the device. The formulae are tested against the performance yielded by a commercially available electronic still camera using a Sine wave, Edge and ISO Standard 12233 methods. Confidence limits calculated using the method detailed by Yeadon, Jones and Kelly and sources of errors are discussed. It is suggested that the presented formulae provide a better indication of discrete array performance for inclusion in system design for critical applications.