Evaluating the patient dose or exposure parameters considering the image quality can improve the chances of accurate diagnosis and reduce the unnecessary exposures from medical devices such as mammography. This study aimed to evaluate digital and conventional mammography machines while considering the trade-off between image quality and mean glandular dose (MGD) using a phantom. In the present study, one full-field digital mammography (FFDM) and two film-screen mammography (FSM) machines were investigated. The MGD values and image quality were assessed using the American College of Radiology (ACR) phantom at various mAs and constant kVp values. The results were obtained and compared with European guidelines. Friedman and Wilcoxon statistical tests were used to show the comparison. The results from the quality control (QC) tests demonstrated that all machines are functioning well. The best image quality in the digital mammography machine was observed at the MGD of 1.8 mGy and 55 mAs. In addition, the two conventional machines had the best image quality regarding the imaging of the ACR phantom at 65 mAs with an MGD of 2.1 mGy. These values were considered as appropriate values for the studied mammography systems. Furthermore, the Friedman test demonstrated that there are significant differences between the measured image quality values obtained from the different machines (p < 0.05), however, according to the Wilcoxon test there were not any significant differences between the conventional machines at various mAs values. Owing to the results, for a medium breast size, the image quality will not be improved with increasing the exposure after a specified MGD corresponds to a certain mAs. It is notable that this value is smaller in digital mammography system at a reasonably low dose.