What is the numerical aperture of a lens directly related to?

The numerical aperture (NA) of a lens is directly related to the lens's ability to gather light and resolve fine specimen detail at a fixed object distance. It is defined by the formula NA = n × sin(θ), where 'n' is the refractive index of the medium in which the lens is working (usually air or immersion oil) and 'θ' is the half-angle of the maximum cone of light that can enter or exit the lens. A higher NA indicates better resolution and light-gathering capability.