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Camera Lens News No. 8
Fall 1999 |
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What is so special about the Zeiss Superachromat lenses?
A beam of white light travelling through a glass prism becomes dispersed into a spectrum showing all colors of the rainbow.
This is because glass, when deflecting light rays, does not apply the same degree of deflection to every color. Basically the same happens when white light beams travel through a lens element. (What appears to the human eye as white light is an even range of radiation encompassing all wavelengths from 400 nanometers (nm, one billionth part of a meter) to 700 nm, where 400 nm represents blue color and 700 nm represents red).
As a consequence of this varying deflection a single lens element would focus different colors of a subject to different foci. Thus a film frame could not record a perfectly sharp picture of a subject for more than one color at the same time. One could only focus perfectly for a single color in a picture, and all others would turn out to be more or less unsharp.
To overcome this problem the ”Achromat” lens design was invented. This is a combination of different lens element curvatures and glass types, that can perfectly focus 2 colors in the same plane. All other colors are still slightly out of focus, but to a far lesser degree compared to the pictures from the single lens element. These out-of-perfect- focus components of light are called ”secondary spectrum”. The visual appearance of a secondary spectrum, if sufficiently enlarged, is unsharp contours and color fringes. However, the Achromat design enables aberrations, including chroma, to be corrected to such a high degree that it has served as basis for almost all camera lenses.
There is, however, a way to improve the chromatic characteristics of lenses: In 1886 Ernst Abbe, the scientific genius of Carl Zeiss, found out that calcium fluoride crystals of extreme purity, if used for lens elements in a well-balanced combination with at least two other thoughtfully chosen types of optical glass, allow to design lenses that can even focus 3 different colors in the same plane at the same time. He called his invention the ”Apochromat”. Zeiss apochromatic lenses marked Zeiss’s clear superiority in design and manufacture of microscopes. They enabled quantum leaps in medical science and related fields, where powerful microscopes were crucial for success.
Apochromatic camera lenses later became the indispensible tool for the pre-press color separation business. Also, apochromatic correction is very useful for improving the performance of long focal length lenses, especially true telephoto designs, since their image quality is usually limited by color aberrations, the inability to perfectly focus different colors of a subject in the same plane at the same time. With an apochromatic correction the performance of a very long telephoto lens can be – as a rule of thumb – approximately doubled over that of an only achromatic version of the same lens, since the secondary spectrum can be reduced to half the amount of the respective achromatic design.
Zeiss found a way to even improve on apochromatic performance: the superachromatic correction. This is a lens design that can even focus 4 different colors perfectly in the same plane at the same time. But not the 4 instead of 3 (compared to the apochromat) is the important improvement here, rather the largely reduced secondary spectrum of a Superachromat. Compared to an achromatic correction a superachromatic correction can improve the secondary spectrum by factors up to 10! Superachromat lenses offer a stunning optical performance, brilliance, sharpness and freedom from color fringes.
Manufacturing Superachromt lenses is extremely demanding in terms of optical materials, mechanical precision, optical metrology, and worker devotion and skills. Superachromatic lenses are truly a class of their own. Currently three Zeiss Superachromat lenses are available, all in the Hasselblad medium format SLR camera system:
• Superachromat 5,6/250,
• Tele-Superachromat 5,6/350, and
• Tele-Superachromat 2,8/300.
The two f/5.6 versions are equipped with a Prontor CFi central shutter, the fast f/2,8 Tele-Superachromat is de-signed for use with focal plane shutter Hasselblad cameras. Adapters are available (e.g. from Kyocera/Contax, Novo-flex, and others) to use these lenses with Contax and many 35 mm SLR cameras plus the new Contax 645 medium format SLR. | |
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