Focal point (focus) is the point at which light rays converge after passing through the lens. The focal point is described as a "point", but due to the nature of light waves, it is not strictly an infinitesimal point. When multiple rays are geometrically focused at one point, the light wave is diffracted and interferes with each other. As a result, the energy (or intensity) of the light exists with an intensity distribution in a finite region of about the wavelength scale. This intensity distribution determines a size of the focal point.
The light intensity distribution near the focal point and its magnitude influences the performance of technologies that utilize the focal point of laser light, such as laser microscopes and laser processing machines. The theory of the intensity distribution near the focal point has been established using wave optics, and methods for controlling the intensity distribution and various measurement and analysis methods using the focal point have been developed. However, since a perfect lens or optical system cannot exist yet, the light intensity distribution of the focal point generated by an actual lens deteriorates compared to the ideal focal point (theoretical value). Therefore, lenses optimized for various purposes (limitation of usage conditions and pursuit of required performance) are designed and manufactured and are used in various devices.