One important factor in the resolution available from a lens system is it's size. The NA is derived from the size of the lens, its working distance, and it's index of refraction. High-quality objective lenses indicates their Numerical Aperture on the sides of their barrels. In general, the useful magnification of an objective is 1000 times its Numerical Aperture. Primarily due to Optical Aberrations , actual resolution will be less than what the calculations predict.
Aberrations are optical imperfections that impair the resolution performance of a lens.
The shorter the wavelength of light is, the more it will be refracted by an optical surface. As a result blue light has a shorter focal length then red light. The picture shows a lens focusing a white light point source. At the point where green light is focused, the red and blue light is blurred. Spherical aberration occurs when the edges of a lens refract more light than the center.