Nikon’s Extra-low Dispersion (ED) and Super ED glass help correct chromatic aberrations, or optical color defects, caused when different light wavelengths do not converge at the same point after passing through optical glass. Calcium fluorite crystals were once used to correct this problem in telephoto lenses, but the substance cracked easily and was sensitive to temperature changes. So Nikon created ED glass, which offers all the benefits, but none of the drawbacks of calcium fluorite-based glass. ED glass is now an essential element in NIKKOR’s telephoto lenses, helping deliver stunning sharpness and contrast, even at maximum aperture.