  Our ability to optically interrogate nanoscopic objects is controlled by the
difference between their extinction cross sections and the diffraction limited
area to which light can be confined in the far field. We show that a partially
transmissive spatial mask placed near the back focal plane of a high numerical
aperture microscope objective enhances the extinction contrast of a scatterer
near an interface by approximately $T^{-1/2}$, where T is the transmissivity of
the mask. Numerical aperture based differentiation of background from scattered
light represents a general approach to increasing extinction contrast and
enables routine label free imaging down to the single molecule level.
