The objective lens consists of two or more lens elements in an achromatic or apoc-hromatic configuration. The achromatic doublet is the most common lens in "standard" binoculars, but high-quality binoculars, particularly large astronomical
Figure 2.1. Light-path through prismatic binoculars.
binoculars, may have an apochromatic triplet. There may also be additional lenses to correct for other optical aberrations such as spherical aberration (SA), coma, or field curvature. Achromats bring two wavelengths (colors) of light to the same focus. A simple achromatic doublet would have a biconvex element of crown glass in front of a weaker diverging element of flint glass. Modern achromats may use special glasses, such as extra-low dispersion (ED) glass, in order to give better color correction. Apochromats, which bring three wavelengths of light to the same focus, may employ expensive (but brittle) fluorite glass.
Large aperture astronomical binoculars have objectives of relatively small focal ratio, often as small as f/5, and sometimes less. An achromatic doublet of 100-mm aperture with a focal ratio of f/5 will have significant chromatic aberration, especially off-axis, no matter which glasses are used. This can be particularly obtrusive on bright objects, such as the Moon or the naked-eye planets. Even a fluorite apoc-hromat of this aperture and focal ratio will show off-axis false color on these objects.
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