Microscope is based on 4-f system which consist of two lenses; an objective lens and a tube lens. By two lens, the magnification is determined
$$ M=\frac{f_{\rm tube}}{f_{\rm obj}} $$
where $f_{\rm obj}$ and $f_{\rm tube}$ are the focal lengths of the objective lens and tube lens respectively.
Typical focal length of the tube lens Thorlabs, Nikon, Leica, Mitutoyo: $f_{\rm tube} = 200~{\rm mm}$ Olympus: $f_{\rm tube} = 180~{\rm mm}$ Zeiss: $f_{\rm tube} = 165~{\rm mm}$
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_ID=5834
For $f_{\rm tube} = 200 ~ {\rm mm}$ tube lens, 4X: $f_{\rm obj} = 50 {\rm mm}$ 10X: $f_{\rm obj} = 20 {\rm mm}$ 50X: $f_{\rm obj} = 4 {\rm mm}$ 100X: $f_{\rm obj} = 2 {\rm mm}$
Numerical aperture is the
$$ ({\rm NA}) = n \sin \theta $$
$n$ refractive index of immersion medium
$\theta$ half angle of the light cone
Under the paraxial approximation ($\le 15{^\circ}$)
$$ ({\rm NA}) = \frac{(D/2)}{f} $$
$D$ diameter of the lens
$f$ focal length
$$ w_0 \rightarrow \frac{\lambda}{2({\rm NA})} \\ z_R = \frac{\pi w_0^2}{\lambda} $$