Abbe Limited Farfield Focus

Without recording significant signal from a sample's nearfield - i.e. the non-propagating, "evanescent" components - no imaging technique stands a chance to resolve any lateral feature beyond a limit which is proportional to the observation wavelength. The proportionality factor depends slightly on specific circumstances, but lies typically between ~0.5 and ~0.1. This "failure" of propagating-waves-only techniques can be traced to the loss of lateral information encoded in the high-frequency part of the lateral Fourier spectrum, which is associated precisely with the evanescent waves.

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