Archived posting to the Leica Users Group, 2001/01/06
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The Soviet Jupiter-9 has that kind of glass. Rob McElroy wrote: > Martin, > > In terms of lens design, "apochromatic" refers to a lens in which the primary colors of light are all focused at the same plane (the film plane). The > term has nothing to do with the inherent color of the glass itself, only to the lens's ability to focus the entire visible spectrum accurately (all > three primary colors). > > Most lenses are achromatic and only bring two of the primary colors to a precise focus at the film plane. One color, usually red I believe, focuses > just a bit behind the film plane. > > If the optical designer of a given lens, designs it so that all three primary colors focus at the exact same plane, then the lens is appropriately > designated as apochromatic. Traditionally only symmetrically designed process lenses with very slow speed could technically and appropriately be > classified as apochromatic. > > Over the past 20 years or so, many camera and lens companies began to label some of their best lenses as "apo" or "apochromatic", even though most of > them were not truly apochromatic. Improving the lens design and reducing chromatic aberrations made them closer to apochromatic, but the "apo" label > was erroneously applied more as a marketing tool, than as an actual claim of focusing perfection throughout the entire visible spectrum. Just because > a lens is great does not make it apochromatic. > > The Apo-Lanthar lenses made by Voigtlander for large format cameras in the 50's and 60's, were some of the first lenses to make use of a > highly-refractive rare-earth glass called lanthanum which has a slightly warm/yellow color, which can be clearly seen if you just pick up the lens and > look through it. It's actually quite pleasing. Like putting a very slight warming filter over your lens. > > Hopefully that was clear. > > Regards, > Rob McElroy > Buffalo, NY > > Martin Howard wrote: > > > Rob McElroy jotted down the following: > > > > > Do you know if the new Voigtlander 90mm/f3.5 Apo-Lanthar is using the > > > lanthanum glass that the original 1960's large format Apo-Lanthar lenses used? > > > The sharpness and contrast of these large format lenses is quite legendary but > > > they have a distinct yellow cast, easily visible when looking through the > > > lens. > > > > Could someone please explain to me how a supposedly apochromatically > > corrected lens managed to throw a yellow cast on everything it puts on the > > film? > > > > M.