Filtered backprojection reconstruction is an efficient image reconstruction method which is widely used in CT and 3D x-ray imaging. The way data have to be filtered depends on the acquisition geometry and the number of projections (views) which were acquired. For standard geometries like circle or helix it is known how to effectively filter the data. But there are acquisition geometries for which the application of standard filters yields poor results, e.g. in situations where the number of views is very small or for a limited angular range. In tomosynthesis, both conditions apply, i.e. the number of projections is typically very small and, moreover, the angular coverage is much less than 180°. This paper proposes a new method to design effective filters which are specific for the acquisition geometry. Examples from x-ray tomosynthesis are utilized to demonstrate the excellent performance of the proposed method.