Micro-focus field emission x-ray sources and related methods are provided. A micro-focus field emission x-ray source can include a field emission cathode including a film with a layer of electron field emitting materials patterned on a conducting surface. Further, the x-ray source can include a gate electrode for extracting field emitted electrons from the cathode when a bias electrical field is applied between the gate electrode and the cathode. The x-ray source can also include an anode. Further, the x-ray source can include an electrostatic focusing unit between the gate electrode and anode. The electrostatic focusing unit can include multiple focusing electrodes that are electrically separated from each other. Each of the electrodes can have an independently adjustable electrical potential. A controller can be configured to adjust at least one of the electrical potentials of the focusing electrodes and to adjust a size of the cathode.