The numerical aperture (NA or A) of the objective is a key parameter for the optical image. Essentially, it determines the resolving power of the objective and the brightness of the image.
The objectives are labeled with their particular NA value, for instance 0.9 or 1.4.
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: