The use of small apertures or sharpened tips as sensing elements in scanned‐probe optical sensing devices has led to the development of a number of instruments that provide lateral spatial resolution much finer than that available in conventional optical imaging instruments. Such a device might generally be classified as a scanning optical microscope, or SOM. One particular mode of SOM operation involves the use of a sharpened optical fiber to collect light emanating from a surface. The lateral spatial resolution of such a collection‐mode SOM is discussed in terms of the electromagnetic mode solutions of the probe tip. Numerical results indicate that, though bound modes solutions exist for increasingly fine unclad tips, classical diffraction effects limit resolution to a finite fraction (approximately 1/3) of the source wavelength &lgr;. A second mechanism for signal transduction is shown to involve molecular scattering at the probe tip. An analysis of signal collection efficiency demonstrates that at tip radii below &lgr;/5 for metallic‐clad probes, and &lgr;/10 for probes in a dielectric ambient, scattering dominates and imaging resolution scales with tip size, thus defeating limits imposed by diffraction.