A quantitative description of the deterministic properties of a CT system is necessary when evaluating image quality. The most common such metric is the modulation transfer function (MTF), usually calculated from a line spread function (LSF) or point spread function (PSF). Currently, there exist many test objects used to measure the LSF or PSF. In this paper we report on a comparison of these measures using: a thin foil slit test object, a teflon cube edge test object and a novel "negative" cube test object. Images were acquired using a custom-built bench-top flat-panel-based cone-beam CT scanner and a cylindrical water-filled PMMA phantom with the test objects embedded in the middle. From the 3-dimensional reconstructed volumes, we estimated the LSF either directly or as estimated from the edge spread function. From these, a modulation transfer function can be estimated, and the frequency dependent image transfer of each object can be reported.