The influence of aperture diameter on image contrast and resolution in scanning near-field optical microscopy has been investigated. Evanescent standing wave fields at glass–air interfaces were probed with well-characterized aluminum-coated fiber tips. A strong decrease in image contrast was found when the aperture diameter exceeds a critical value. Numerical simulations with the concept of an intensity transfer function using the axial component of the Poynting vector inside the probe demonstrate that this phenomenon is the result of the increasing influence of higher-order waveguide modes on power transmission through the probe. It has been shown that standing wave patterns are convenient and reliable reference devices for individual probe characterization and quantification of their resolving power. ©1997 American Institute of Physics.