Final answer:
Focusing a beam affects the length of the near field due to diffraction in wave optics. The focal point spreads to become a focal spot with increasing intensity as the numerical aperture (NA) increases. The higher the NA, the greater the chances of photodegrading the specimen.
Step-by-step explanation:
When focusing a beam, the length of the near field is affected by diffraction. In geometric optics, the focal point is considered an infinitely small point with high intensity. However, in wave optics, due to diffraction, the focal point spreads to become a focal spot with a decreasing size as the numerical aperture (NA) increases. With a higher NA, the intensity in the focal spot increases, increasing the chances of photodegrading the specimen.