Archived posting to the Leica Users Group, 2001/01/06
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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.