The lateral resolution of a microscope is the minimum distance between two points, which still makes it possible to distinguish them; if the distance between the two points is less, they merge into one.
In the case that the instrument is based on the use of radiation with its own associated wavelength, such as traditional optical microscopes, resolution and wavelength used are closely related parameters.
As a first approximation, and not taking into account optical aberrations, we can consider that the relation that binds the lateral resolution (d, ie distance between two resolved points), N.A. the numerical aperture of an optical system (the whole system) and λ the wavelength of the radiation used) is:
d = 0.61 λ / N.A.

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