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For linear light scattering in apertureless scanning near-field optical microscopy, we have studied the correlations between the tip radius of the probe, signal strength, spatial resolution, and sample material. Pronounced variations of the near-field distance dependence on tip shape and dielectric function of the sample are observed. For very sharp metal tips, the scattered near-field signal decays on a 5 nm length scale. Despite this highly localized tip–sample coupling, the contrast is found to depend sensitively on the vertical composition of the sample on a length scale given by the penetration depth of the incident light. The resulting implications on the use of the technique as an analytic probe method are discussed. © 2003 American Institute of Physics.