Final answer:
Focal spot size in Physics affects the intensity and resolution at the focus point of a lens; the smaller the focal spot size the higher the intensity and potential for photodegradation which increases with the lens's numerical aperture.
Step-by-step explanation:
The question What does focal spot size effect? pertains to Physics, exploring concepts related to wave optics and the physical limitations imposed by diffraction on the focusing properties of lenses. In geometric optics, which is an idealization, the focal point is considered infinitesimally small. However, wave optics accounts for the diffraction of light which causes the focal point to spread out into what is known as a focal spot. As the numerical aperture (NA) of the objective lens increases the size of the focal spot decreases leading to higher intensity within that spot.
This increment in intensity augments the likelihood of photodamage or photodegradation of the specimen being observed. Despite the changes in NA, it is important to recognize that the focal spot cannot become a true point due to the inherent nature of light diffraction. The significance here lies in understanding that as the NA increases, not only does the resolution improve, but also the chance of damaging the specimen through intense light exposure increases, which is a critical consideration in fields such as microscopy and optical engineering.