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A new measurement technique is proposed for determining the second Petermann definition of mode field diameter (MFD) for single-mode fibers with both near-Gaussian and non-Gaussian radial field distributions. The method involves measuring the power passed through a circular aperture as it is scanned along the far-field optical axis of a fiber. Using this technique, it is possible to select those acceptance angles that are most critical to the accurate determination of MFD for a particular fiber design. Also, the aperture can be positioned close enough to the fiber endface so that collection of the total far-field power is assured. Measurements of MFD for conventional and disperson-modified single-mode fibers have been made at 1.3 μm and 1.55 μm with a precision of +/-0.05 μm and a comparison with the far-field angular scanning method gives agreement ot within 1 percent for both fiber types.