The present invention provides a printing system with an apparatus for defect detection comprising a sensing unit for sensing a surface geometry or topology of a sheet and for generating data representative of that surface geometry or topology, a processor device for processing the data to detect and classify deformations, and a controller for controlling further progress of the sheet along the transport path of the printing system in dependence upon the deformations detected and classified by the processor device. Furthermore, the invention provides a corresponding method of detecting defects in a printing system.