ED (Extra Low Dispersion) Glass
From Nikonians Wiki
A glass developed and trademarked by Nikon Corporation, used in telephoto lenses to offer superior sharpness and color correction by minimizing chromatic aberration. These lenses are resistant to temperature changes, preventing focus shift problems in lenses that use calcium fluorite crystal elements. Fluorite cracks easily and is sensitive to temperature changes that can adversely affect focusing by altering the lens’ refractive index. Super ED glass is a new type, used together with ED glass in some lenses achieves an even higher degree of freedom from chromatic aberration.
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