Numerical Aperture (N.A)

Object side N.A.” and image side N.A.’.
The formula is established in the figure below, where u is the half-angle that object size entrance pupil extends to the object side, u’ is the half-angle that the exit pupil extends to the image side, and n is the refractive index on the object side and n’ is the refractive index on the image side.
NA = sin u x n
NA’ = sin u’ x n’
For finite lenses, it can be calculated by NA=M/(2xF) and NA’=1/(2xF) in relation to W.F/#.
The relationship between NA and NA’ can be expressed as NA=NA’x Opt. Mag or NA’=NA/Opt. Mag.