In situ three terminal electron field emission characterization of an isolated multiwalled carbon nanotube has been performed, where both anode and gate electrodes are attached to high precision piezodrivers. All measurements are performed in a scanning electron microscope allowing accurate knowledge of the local environment of the nanotube to be obtained. It is shown that the presence of the grounded gate electrode screens the applied field by approximately 32%. This technique in positioning the gate and anode electrodes allows for an estimate of the gate transparency factor and demonstrates characterization of individual carbon nanotubes without the need for fabrication of arrays of emitters.