Aspherical, Fluorite or 'L' Lenses
An enhanced range of FD lenses was available for photographers who required the highest optical and mechanical performance. In addition to more robust mechanical construction, these lenses used a variety of special technologies, including ground aspherical surfaces, calcium fluorite optical elements, and ultra-low-dispersion glass. Canon used these means to achieve outstanding optical performance at the extremes of lens design: wide apertures and extreme focal lengths. Aspherical surfaces improved performance of wide-angle and standard lenses at very wide apertures. Ultra-low dispersion and fluorite elements virtually eliminated chromatic aberration at long focal lengths.
The earlier versions of these lenses are designated "AL", "Aspherical", or "Fluorite" on the front of the lens. The post-1979 'New FD' versions acknowledged all the exotic technologies under the single designation "L" (popularly said to indicate 'luxury' or 'asphericaL'). Canon continued the "L" designation, and the famous red ring around the lens front, in the current EF autofocus lenses for EOS cameras, where the symbol now officially stands for "Luxury."
Read more about this topic: Canon FD Lens Mount
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