We report on the results of a combined experimental and computational investigation on the properties of amorphous carbon nano-tips grown by carbon contamination in a scanning electron microscope and used as electron field emitters. We have calculated the strength of the electric field at the emitter surface F-tip and the dimensionless enhancement factor gamma (= F-tip/F-m) for various geometries and sizes by means of the finite element method. By using F-tip, experimental data and results from the Fowler-Nordheim plot, we have found consistent values of the work function, the surface emitting area and the current density J. The values of F-tip and J are in reasonable agreement with other experimental deductions.