Imaging resolution with laser 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?
 A: When you are limited by the size width of the laser beam, i.e. you are not limited by the diffraction of the lens you are going throught, the relation between the waist of the "beam image" $w_0^{"}$ and the waist the "beam object" $w_0$ is given by the relation:
$$\frac{w_0^{"}}{w_0}= \frac{\lambda f}{\pi w_0}$$
You can prove this easily with the lens formula for gaussian beam from this article (S.A. Self, Focusing of spherical Gaussian beams and taking the case where the beamimage waist is at the focal point of the lens)
So to answer your question. It's depend on which aperture is limiting your spot size: the lens or the beam width
A: 1-minor point: laser illumination will create speckle patterns and ruin your image
2-if the laser illuminates the object then you'll get diffuse reflection and also transmission if the object is thin
3-diffuse reflection will shoot light in all directions so the effective NA of your lens wont change
4-transmitted light will have a much lower NA (because it's parallel) so if you were in a transmitted light imaging setup (for ex laser illumination vs kohler illumination on a microscope) then yes NA will change. 
