The transverse optical resolution of a lens, as determined by its point spread function, can be approximated as $w$ ~ λ/(2$NA$), where $NA$ is the numerical aperture and λ is the wavelength of the light. The $NA$ is ~ $D$/(2$f$), where $D$ is the lens diameter and $f$ is the focal length. However, what if I have a laser beam illuminating an object, and the beam diameter is less than $D$? Would I have to change $D$ in this equation to approximate the resolution?