 Numerical aperture

The numerical aperture is $n\sin \alpha$, where $n$ is the refractive index of the medium between object and objective, and $\alpha$ is the half-angle of the admitted cone of light. Specify object-side or image-side numerical aperture; the microscope convention usually means the object side.^[ref]

Numerical aperture (NA) is a dimensionless figure that characterizes the range of angles over which a lens can accept or emit light. It is a critical parameter in microscopy and optics, particularly when discussing the resolution and light-gathering ability of lenses.