The modulation transfer function (MTF) is the indicator most frequently used for image resolution assessment. Traditionally in photogrammetry, the MTF is directly estimated using specifically designed targets such as the USAF 1952 three-bar resolution test target, but it can also be indirectly derived from the system degradation function known as the point-spread function (PSF). Theoretically, MTF and PSF are related through a spatial-frequency domain transformation (a Fourier transformation), meaning the same attributes are expressed in different domains. When using real images, however, it is not clear whether these two functions produce the same result. In this study, both spatial and frequency domain approaches were experimentally compared through simulations. Results show that the MTF derived from the PSF has better accuracy than an MTF estimated directly from the resolution target.