The numerical aperture (NA or A) of the objective is a key parameter for the optical image and determines the resolving power of the objective and the brightness of the image. It is defined by the sine of the half aperture angle a of the lens and the refractive index n of the immersion medium. According to this definition, the larger the numerical aperture, the more narrow the focal spot and hence the higher the resolving power.
The objectives are labeled with their magnification, followed by the particular NA value, for instance 10x/0.40 or 63x/1.40.
The numerical aperture of the objective can be changed by using iris diaphragm objectives.
The term ‘numerical aperture’ is explained in more detail in the Leica Science Lab: