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We present an exact calculation of Maxwell's equations for a tip-metallic interface configuration by using a nonlocal boundary condition. The system is two dimensional, and consists of a cylinder at subwavelength distance from a corrugated surface of Ag or W. Both media are described by a frequency-dependent complex dielectric constant. The plasmon intensity has a huge enhancement, which is maximum when the tip-surface distance is about 1 nm, and it is sensitive to variations of 0.1 nm. This can account for recent experimental data on atomic resolution in photon emission by scanning tunneling microscopy operating at a constant current mode by Berndt et al.

Recent experimental work has shown that the contrast of near-field optical images depends on the path followed by the tip during the scan. This artifact may misguide the interpretation of the images and the estimation of the optical resolution. We provide a rigorous theoretical study of this effect based on three-dimensional perturbation theory and two-dimensional exact numerical calculations. We quantitatively study the dependence of the artifact on the illumination/detection conditions and on the scattering potential of the sample. This study should provide guidelines for future experimental work.

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