The far field angular intensity distributionI(&thgr;) of the &lgr;=633 nm radiation transmitted through diffracting circular apertures is measured for diameters ranging between 60 and 500 nm. The circular apertures are located at the apex of aluminum coated tapered optical fiber tips.I(&thgr;) depends sensitively on the aperture diameters down to &lgr;/6. This property is used to determine the optical aperture size of metal coated tapered optical fiber tips used for near field scanning optical microscopy. ©1995 American Institute of Physics.