Digital filter multiple access (DFMA) passive optical networks (PONs) are proposed and investigated, where digital signal processing (DSP)-enabled, software reconfigurable, digital orthogonal filtering is employed in each individual optical network unit (ONU) and the optical line terminal (OLT) to enable all ONUs to dynamically share the transmission medium under the control of the centralized software-defined controller and the transceiver-embedded DSP controllers. The DFMA PONs fully support software-defined networking (SDN) with the network control further extended to the physical layer. As digital filtering is the fundamental process at the heart of the proposed DFMA PONs, the filtering-induced trade-offs among upstream transmission capacity, filter design flexibility and filter DSP complexity are examined, based on which optimum filter design guidelines are identified for various application scenarios. Furthermore, the performance characteristics of the DFMA PONs are also numerically explored in terms of maximum achievable upstream transmission capacity and ONU count-dependent minimum received optical power