Assessing the image resolution of the differential phase (dP) and the dark field (DF) image in grating-based phase-contrast imaging (GB-PCI) requires a precision machined wedge and edge respectively in order to generate an accurate edge spread function (ESF) from which the modulation transfer function (MTF) can be calculated. Imperfect test objects, i.e. truncated wedges (dP), unsharp or too thick edges (DF), can lead both to an over-or an underestimation of image sharpness, making the MTF potentially test object dependent. Here the object free method of Kuhls-Gilcrist, which resolves the MTF from noise power spectra (NPS method), is applied to the transmission, dark field and differential phase image in GB-PCI. In parallel to the NPS method, each MTF was also determined using a test object based approach. Good agreement was found between both approaches. Moreover, it was possible to distinguish unsuitable test objects and this work illustrated that selecting DF test objects can be difficult. This sharpness measurement method offers a robust alternative to test object approaches and can be used to verify sharpness of test objects or to generate an accurate MTF if no such object is available.