Numerical aperture is a measure of the acceptance angle of an objective and can also be considered as the ability of the objective to gather light and resolve specimen detail at a fixed objective distance. It is defined by the expression: NA = n(sin(a)), where NA is numerical aperture, n is the refractive index of the medium between the objective front lens and the specimen, and a is the half angle aperture of the objective.
The thickness of an optical section is predominantly determined by the numerical aperture of the objective and the diameter of the confocal pinhole (and, to a lesser extent, the wavelength of the light.
Koehler illumination was first introduced by August Koehler in 1893 to provide optimal contrast and resolution in light microscopy that complement the numerical aperture of the objective lens.