Interferometers for the measurement of topography rely on imaging systems to map surface points to a camera. The response of an interferometer to variations in surface height depends on the filtering properties of the imaging system. Here we provide a simple and practical model of an imaging interferometer using classical Fourier optics, including the effects of partial coherence. The model is useful for understanding basic properties such as lateral resolution and error sources related to measurement principles that make assumptions regarding the fidelity of the instrument response over a range of surface spatial frequencies and light source wavelengths.