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Microscopy is fundamentally limited by the wavelength of the illumination.
The formal expressions presented by Abbe for lateral and axial resolution in the optical microscope are:
Resolutionx,y = λ / 2[η • sin(α)](1) Resolutionz = 2λ / [η • sin(α)]2(2)
where λ is the wavelength of light (excitation in fluorescence), η represents the refractive index of the imaging medium, and the combined term η • sin(α) is known as the objective numerical aperture (NA).