John M. Lovda wrote: > This is a very basic question I have > never gotten a good technical answer for. > > What makes UD glass so special or expensive? > Other L lens attributes such as aspherical or > flourite lenses are intuitively obvious since > the grinding, molding, deposition or > crystal growing techniques are more technically > complicated than a regular spherical glass lens. > What's the deal with low dispersion glass; > do the rare earth additives cost a million > dollars an ounce? > Is the glass so hard it takes longer to grind? > Is it really fragile and hard to handle with > lots of scrap? Why can Tokina, Sigma or > Tamron put UD glass into a much cheaper lens? > > Any optical experts out there? ------------------------------------------------------------------
Well John, since no one answered you directly I will try to add my two cents. UD glass is one acronym given to (Ultra) Low Dispersion glass. Tokina calls it SD for Super Low Dispersion glass, and others have their own acronym. All transparent materials are dispersive and have what is known as a refractive index. This increases as you go toward the blue spectrum. Noting this, the blue part of a ray of light will bend or reflect when it strikes a glass surface, resulting in "Chromatic aberration." It was discovered years back that if you had a weak negative lens of high dispersion glass element and mate it with a stronger lens of "low dispersion" glass you would eliminate chromatic aberration forming an achromatic lens. Achromatic lenses basically bend the light rays back onto themselves and the colors of the spectrum combine at a common focus point, in our case the film plane. Contrary to popular belief using Low dispersion glass (and it does not have to be the most expensive kind) can make designing an inexpensive lens easier with fewer elements needed to correct color. This can keep prices down and provide a higher performing lens at a lower cost. On the other hand, certain glass types with specific refractive indices can be very expensive to make. Canon uses various types in their L lenses and this is "one" of the reasons for the higher prices with L lenses, the glass is more expensive and the refractive index varies so they have to make small batches of specific glass for specific use. BTW, Canon, Nikon, Tamron, and Cosina make their own glass. Sigma, Tokina, Pentax, and in some instances Leica use glass made by Hoya (world's largest glass manufacturer). To dispell another myth, sometimes German lenses will also Japanese glass. Tamron supplies glass to many including Schneider. Hope this helps. Peter K * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
