The effective amplitude of the interference pattern produced by sending a laser beam back upon itself in a BK-7 glass prism was measured in the evanescent wave region outside the prism using a tapered fiber tip scanned and retracted piezoelectrically. This effective amplitude was related to the effective aperture size of the fiber by assuming that the finite extent of the fiber averaged the optical signal over a Gaussian distribution. In this way the resolving power of the photon scanning tunnelling microscope was determined operationally. Combining this information with electron microscopic imaging of the fiber tip leads to the conclusion that upon heating and drawing the fiber, its cladding decreases in size more rapidly than the core so that final ratio of core diameter to outer diameter in the tapered fiber is approximately twice the original ratio.