Electron field emission from an isolated carbon nanotube (CNT) was performed in situ in a modified scanning electron microscope, over a range of anode to CNT tip separations, D, of 1–60 μm. The threshold field required for an emission current of 100 nA was seen to decrease from a value of 42 V μm−1 at an anode to CNT tip separation of 1 μm, asymptotically, to approach 4 V μm−1 at a separation of 60 μm. It is proposed that at low D, the electric field enhancement factor (β) reduces as the anode electrode approaches the CNT mimicking a parallel plate configuration. Under “far field” conditions, where D>3 h, where h is the CNT height, the CNT enhancement factor is no longer dependant on D, as shown by the asymptotic behavior of the threshold field, and is purely a factor of the CNT height and radius. For each CNT to tip separation, measured emission current data together with the threshold field and enhancement, are consistent with a Fowler-Nordheim analysis for the far field conditions, and dispels the need for a novel emission mechanism to explain the results as has been proposed recently.