There are two types of optical units. Radial optical units are measured in the image plane, and axial optical units are used to measure distances between the image plane and the observer.

The number of optical units ${\displaystyle v}$  in a given radial length ${\displaystyle r}$  is given by:

${\displaystyle v_{\mathrm {radial} }={\frac {2\pi }{\lambda }}{\frac {n\sin \alpha }{M_{\mathrm {tot} }}}r}$ 

where:

• ${\displaystyle \lambda }$  is the wavelength
• ${\displaystyle n\sin \alpha }$  is the numerical aperture
• ${\displaystyle M_{\mathrm {tot} }}$  is the total magnification

Axial optical units are more complicated, as there is no simple definition of resolution in the axial direction. There are two forms of the optical unit for the axial direction.

For the case of a system with high numerical aperture, the axial optical units in a distance z are given by:

${\displaystyle u_{z}={\frac {2\pi }{\lambda }}{\frac {(n\sin \alpha )^{2}}{\eta }}z}$ 

where:

• ${\displaystyle \eta }$  is the index of refraction of the medium above the optical plane.^[2]

For systems with low numerical aperture, the axial optical unit is:

${\displaystyle u_{z}={\frac {8\pi \eta }{\lambda }}\sin ^{2}({\frac {\alpha }{2}})z}$