Spherical aberration in light microscopy
The curved surface of a lens is most often spherical (this shape is easy to make). When light refracts through a lens with a spherical surface, the rays that pass through the outer diameters of the lens will focus at a different point from rays that pass through the center. This causes the resulting image to be blurred.
When light is refracted from the outer vs. inner portions of the curved surface of a spherical converging lens (A) or diverging lens (B), the resulting image is blurred along the optic axis (i–ii).
The simplest method of reducing spherical aberration is to place an aperture, hole or "stop", in front of or after the lens. The aperture blocks out rays that blur the image. The disadvantage is, however, a greatly reduced light throughput of the lens.
This is an objective from a microscope from the 1760s. The objective lens is at the bottom. To reduce spherical aberration, the manufacturer used the small aperture (black, left) so that only light that passes through the center of the lens would be used in image formation.