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Glossary terms about Numerical Aperture

Numerical Aperture (NA)
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.

20 pages mention Numerical Aperture

Collecting Z stacks
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. 
Components of the confocal microscope
Confocal microscopy
Detection parameters
Fluorescence microscope
Fluorescence microscopy
Image acquisition (single scan)
The most important quality of a lens is not its magnification, essential though that is, but its numerical aperture
Light microscopy
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. 
Multichannel imaging
Objective lens - magnification versus resolution
Pinhole - effects of resolution
Pixel arrays - dimensions versus pixel size
Practical image acquisition
Scan speed - pixel dwelling, saturation
Scanning and resolution
As was mentioned in the Introduction it is the numerical aperture (NA) that is most important in resolution not the magnification. 
Sequential and simultaneous
Transmitted light microscope