The angular resolution may be converted into a spatial resolution,by multiplication of the angle (in radians) with the distance to the object. For a microscope, that distance is close to the focal length f of the objective.
The factor 1.220 is derived from a calculation of the position of the first dark circular ring surrounding the central Airy disc of the diffraction pattern.
This is the size, in the imaging plane, of smallest object that the lens can resolve, and also the radius of the smallest spot to which a collimated beam of light can be focused.Related formulas
|Δl||Spatial resolution (m)|
|f||Focal length of the objective (m)|
|D||Diameter of the lens' aperture (m)|