Numerical Aperture is a dimensionless value representing the Light Gathering Power of a microscope objective lens.

NA was first described by Ernst Abbe, and is defined:

where:

n = the lowest refractive index between the condenser and objective;

a = the angular aperture, or collection angle, of the objective.

The common element is that resolution in a microscope system is a function of the NA of the objective and the wavelength of light used for observation. Rayleigh "improved on Abbe's definition by accounted for the diffraction of light around a small object, and described resolution as the minimum distance between two (light diffracting) points.

Rayleigh's definition of resolution may be described in the following two ways depending on whether light is transmitted through the sample or light is reflected off the sample.

Brightfield microscopy
(transmitted light)
Fluorescence microscopy
(self-luminous object)
Objectives with different Angular Apertures (a) and Working Distances (WD). Objective A will have a higher NA due to the increased aperture size.
Abbe described a microscope system by defining resolution in terms of wavelength, where d, or the smallest measurable object, is a function of wavelength and objective NA.