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Electron field emission from an isolated carbon nanotube (CNT) were performed in-situ in a modified scanning electron microscope, over a range of anode to CNT tip separations, D, of 1 to 60 μm. The CNT was exposed from a CNT-polymer composite which was physically broken and examined along the broken edge. The threshold field required for an emission of 100 nA was seen to decrease from a value of 45 Vμm-1 at an anode to CNT tip separation of 1 μm, and asymptotically approaches 4 Vμm-1 at a separation of 60 μm. By defining the separation as (D-h) rather than D, where h is the height of the CNT, our applied electric field is E=V/(D-h). Calculation of enhancement factor using the Fowler Nordheim equation shows an increase in enhancement factor with an increasing D, and ties in well with the decrease in threshold field with increasing D. Under "far field" conditions, where D > 3h, the CNT enhancement factor is no longer dependent on D as shown by the asymptotic behaviour of threshold field, and is purely a factor of the CNT height and radius. Fowler-Nordheim analysis allowed calculation of the emission currents for given CNT to tip separation. The calculated emission current, threshold field and enhancement are comparable to that found experimentally.