Purpose: To show that the novel open‐field normalization technique prevents a common error in calculation of the detective quantum efficiency (DQE) caused by zero‐frequency normalization of the modulation transfer function (MTF). Method and Materials: Models describing zero‐frequency and open‐field normalization were used to derive the resulting measured MTF, noise power spectrum (NPS) and DQE using a finite region of interest (ROI) of image data. Simulated one‐dimensional images containing Gaussian blur were used to model a deterministic system and to calculate the resultant values. Measurements were made using both zero‐frequency and open‐field normalization with ROIs ranging in size from 1–10 cm. Results: Use of a finite ROI results in truncation of the system line‐spread function (LSF) causing the zero‐frequency value of the measured MTF to be less than the true MTF value of unity, and causes spectral leakage in both the MTF and NPS. Zero‐frequency normalization of the MTF inflates values at all non‐zero frequencies. Since no zero‐frequency normalization is performed on the NPS, this causes inflated DQE values. Simulated results show a 6% inflation of DQE values for a ROI of 10 cm, which increases as the ROI is reduced. Open‐field normalization accurately determines MTF and NPS (and thus DQE) values at all frequencies away from zero frequency. Conclusion: Open‐field normalization measurements provide a good estimate of the true MTF and DQE. This approach should be used to avoid a common error in DQE calculations that is not obvious and inflates DQE calculations by 5–20%.