θ = 1.22 * λ / D,
where λ is the wavelength of the light and D is the diameter of the lens aperture.
Spatial resolution on the other hand describes the smallest object that a lens can resolve. While angular resolution was employed for the telescope, the following formula for spatial resolution is applied to microscopes.
Spatial resolution: Δl = θf = 1.22 * f * λ / D,
where θ is the angular resolution, f is the focal length (assumed to be distance to object from lens as well), λ is the wavelength and D is the diameter of the lens aperture.
The Numerical Aperture (NA) is a measure of the the ability to of the lens to gather light and resolve fine detail. In the case of fiber optics, the numerical aperture applies to the maximum acceptance angle of light entering a fiber. The angle by the lens at its focus is θ = 2α. α is shown in the first diagram.
Numerical Aperture for a lens: NA = n * sin(α),
where n is the index of refraction of the medium between the lens and the object. Further,
sin(α) = D / (2d).
The resolving power of a microscope is related.
Resolving power: x = 1.22 * d * λ / D,
where d is the distance from the lens aperture to the region of focus.
Using the definition of NA,
Resolving power: x = 1.22 * d * λ / D = 1.22 * λ / (2sin(α)) = 0.61 * λ / NA.