Difference Between Spherical Aberration And Chromatic Aberration
Even the most carefully and exactly made single lens cannot give a perfect image. There are always shortcomings that are troublesome under certain conditions. These basic defects of lenses are called aberrations.
Rays going through the outer parts of a broad lens do not focus at exactly the same point as the rays nearer the axis. The result is that the image is not quite sharp. This effect is known as spherical aberration. The remedy is to block off the outer parts of the lens by using a piece of metal that has a small, round hole in it. A better image is then formed because only the rays coming through the center of the lens are used.
That is what happens when you “stop down” the objective of your camera on a very bright day. This is done mainly to reduce the amount of light that comes in. But it also cuts down spherical aberration. As a result, the image is sharper than it would be with the objective “wide open.” But if there is not much light, stopping down the objective would make the image too weak. Then the only practical remedy is to use lenses that have been ground to a special shape by hand. Such lenses are expensive. They are ordinarily used only in research instruments.
The images seen through a toy telescope or microscope often have annoying colored edges. This is another defect of lenses, called chromatic aberration. Ordinary daylight is really a mixture of all colors of the rainbow. When a ray of daylight enters a lens, the various colors in the light are refracted by slightly different amounts. Each color comes out again as a separate ray going in its own direction. The difference is quite small. But it is enough to make the focal length of a lens noticeably different for each color. This means that there is no one place where all the colors come together to form a sharp image.
Chromatic aberration is often corrected by using a special combination of two lenses in place of one. One of the pair must be a converging lens made of a lightweight kind of glass. The other must be a weakly diverging lens made of heavier glass. Each lens cancels out most of the color-spreading of the other, so that the image is sharp and fairly free of color. Pairs of lenses of this kind are found in optical instruments of high quality. Sometimes sets of three and four lenses are used for even better color correction.
There are several other lens defects that optical engineers must try to reduce or eliminate. In designing a new lens system, they may first make calculations to find which shapes and combinations will do what is needed. Then they trace some rays through the lenses on a large drawing of the setup. By such trials they find a design that makes the lens errors as small as possible. The final test is to grind the lenses, mount them carefully, and try the system under the conditions of actual use.