Final answer:
The smaller focal spot size will produce the best spatial resolution as it allows finer details to be resolved. However, a higher NA leads to smaller spot size but could also cause photodegradation.
Step-by-step explanation:
The focal spot size that will produce the best spatial resolution is a smaller focal spot size. In optics, the focal point of a lens or a mirror is ideally infinitely small, leading to the highest possible spatial resolution. However, due to wave optics and the phenomenon of diffraction, this point actually spreads out to become a focal spot. The size of this spot directly affects the spatial resolution of the image; a smaller focal spot size allows for finer details to be resolved, improving spatial resolution. As the numerical aperture (NA) of a lens increases, the focal spot size decreases, leading to higher resolution and intensity within the focal spot. Nonetheless, it is important to note that there is a trade-off, as very high NA can also lead to photodegradation of the specimen.