What is Hi-UD glass? It is essential to use low dispersion glasses like fluorite (CaF2) or other extraordinary glasses to reduce chromatic aberrations. However, because the refractive index of these special glasses is relatively low, the use of a number of these glasses causes heavier weight for the lenses, though chromatic aberrations are well compensated. Canon has developed a totally new type of glass, "Hi-UD"glass, which stands for High index and Ultra low Dispersion. The Hi-UD has high refractive index and very low dispersion. So, it is possible to correct chromatic aberrations whilst maintaining low weight. Hi-UD glasses are now used for Canon Broadcasting zoom lenses. Hi-UD glass is one of the new technologies to open new possibilities to develop the zoom lenses with low weight and high optical performance.