The spectrum of achromatic lenses
An undistorted view of what an achromatic lens does.
An achromatic lens is a combination of two lenses made of different types of glass, and has considerably less chromatic aberration (false color) than a single lens. All high quality lens systems in modern binoculars, cameras and other optical instruments use achromatic lenses.
A single lens refracts (bends) parallel light and focuses it to a point. The distance of this point from the lens depends upon the curvature of the lens and the refractive index (light-bending power) of the lens material.
A slightly different refractive index applies to each color of light. Consequently when white light, which is a mixture of all colors, passes through a simple lens, the various colors are dispersed and are focused at different points. This produces an image with rainbow-colored fringes, ie with chromatic aberration.
Different types of glass have different refractive indexes, so that one type may be used to compensate for the chromatic aberration of another. It is possible to construct a compound lens (two lenses stuck together) with two types of glass of different curvatures fitted into each other; this is free from chromatic aberration for two colors of light. Crown and flint, which have a different chemical make-up, are the two most commonly-used types of glass.
When white light passes through an achromatic lens corrected for, say, red and blue, there will still be a slight chromatic aberration caused by the other colors, such as green. For high class photographic work, lenses are made with three or more types of glass to eliminate all chromatic aberration for white light. These are called apochromatic or process lenses.
Before the invention of the achromatic lens, by John Dollond in 1758 it was difficult to build powerful refracting telescopes. By using a lens with very shallow curves, chromatic aberration was reduced but at the expense of making the telescope extremely long. Some instruments of this type were made and had to be suspended from towers making them very awkward to use. Sir Isaac Newton, in the 17th Century, believed that it was impossible to overcome the colored fringes produced by lenses, and he invented the reflecting astronomical telescope with a mirror, which reflects all colors equally, instead of a lens.
Even with well-made modern instruments, chromatic aberration is often seen - particularly when looking through a telescope or a pair of binoculars at a dark object against a bright sky. A lens which is well-corrected will show barely-detectable apple-green and plum-red fringes. Poorly-made lenses give vivid blue and orange fringes.
Many spectacles are composites and, therefore, consist of at least one achromatic lens.
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