Numerical aperture, abbreviated as NA.

    The picture shows a Leica 100x objective with a numerical aperture of 0.9. Under normal circumstances, the larger the multiple, the greater the NA value.

    Its size is determined by the following equation: NA=n * sin α， Where n is the refractive index of the medium between the observed object and the objective lens; α Is the objective Angular aperture (2 α） Half of it.

    Numerical aperture is one of the main technical parameters of an objective lens, which determines its resolution. It is directly related to the magnification, working distance, and depth of field of the objective lens. Generally speaking, it is proportional to resolution, proportional to magnification, and inversely proportional to the square of the numerical aperture. As the NA value increases, the field of view width and working distance will correspondingly decrease.