What is Numerical aperture (N.A.)?

Numerical aperture (N.A.): refers to the sine product of the refractive index (h) of the medium between the front lens of the objective lens and the detected object and the half of the aperture diameter (u). 

N.A.=h*sinu/2

The larger the aperture diameter, the brighter the light entering the objective lens, which is proportional to the effective diameter of the objective lens and inversely proportional to the distance of the focal point.

If you want to increase the value of N.A., you must increase the refractive index, so there are oil immersion objectives and water immersion objectives. When the refractive index is greater than 1, the value of N.A. can be greater than 1.

N.A. is proportional to resolution, magnification, and inversely proportional to depth of focus. And when the value of N.A. is larger, the width of the field of view and the working distance are smaller.