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For many photo enthusiasts, Carl Zeiss lenses have long been the ultimate choice. It has been said that the depth and three-dimensionality they deliver is so distinctive that it was possible to tell whether or not an image was produced by a Zeiss lens while watching the image appear during development in the darkroom. The appeal of Zeiss lenses remains unchanged, and many models are available, but the only autofocus Zeiss lenses available for use on digital SLR cameras are those that have been created through close cooperation between Carl Zeiss AG and Sony for the α-series cameras. Below we'll examine three important aspects of Carl Zeiss optical technology. |
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Carl Zeiss |
Ernst Abbe |
No discussion of the development of photographic lenses can be complete without the inclusion of Carl Zeiss AG. It was Ernst Abbe of the Zeiss company who first applied scientific principles to lens design, rather than relying on trial-and-error experience. In fact a significant portion of the history of photographic lens development centers on the Protar, Planar, and Sonnar designs that featured advanced optical paths based on those principles.
The Abbe number used in the evaluation of chromatic aberration, astigmatic lenses that minimize coma and spherical aberration, and apochromatic lenses made of fluorite, are all Carl Zeiss innovations. In many ways the history of Carl Zeiss AG is the history of photographic lenses.
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| Protar® (1890 ~ ) | Planar® (1896 ~ ) | Tessar® (1902 ~ ) | Sonnar® (1929 ~ ) |
Which of the following two photographs appears to be the sharpest to your eyes? The image on the left has relatively low resolution with high contrast, while the image on the right has high resolution but low contrast.
In an era when resolution was considered to be the most important lens specification, Carl Zeiss AG revealed the importance of both contrast and resolution to perceived sharpness and advocated MTF (Modulation Transfer Function) measurements as a means to express that relationship in numeric/graphic form. Carl Zeiss AG was the first to provide MTF data for individual lenses.
Now every lens manufacturer employs MTF graphs to define the performance of their products. The fact that the MTF of each and every Zeiss lens is evaluated before shipping contributes significantly to their consistently outstanding optical performance.
MTF graphs describe the ability of a lens to reproduce the contrast of fine subject details (see the graph on the left, below). Contrast normally decreases as details become finer. MTF graphs provided by most manufacturers are similar to the one on the right, below, in which MTF is plotted for multiple levels of subject detail (expressed as line pairs per millimeter) at a number of points from the optical center of the lens to its periphery.
MTF (Contrast vs. Subject detail) |
MTF (Contrast vs. Distance from lens center) |
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| Spatial Frequency | R | T |
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| 10 lp/mm |  |
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| 20 lp/mm |  |
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| 40 lp/mm |  |
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R: Radial
T: Tangential
The fact that lens coating technology – vapor deposition of a thin, even coating on the lens surface to reduce reflections and maximize transmission – was originally a Carl Zeiss patent is well known. The Zeiss company also developed and proved the efficacy of multi-layer coatings for photographic lenses, and this is the technology that became the T* coating.
Until the introduction of coated lenses, the lens surface would reflect a large percentage of the incoming light, thus reducing transmission and making it difficult to use multiple of elements in lens designs. Effective coatings made it possible to design more complex optics that delivered significantly improved performance. Reduced internal reflection contributed to minimum flare and high contrast.
The Carl Zeiss T* coating is not simply applied to any lens. The T* symbol only appears on multi-element lenses in which the required performance has been achieved throughout the entire optical path, and it is therefore a guarantee of the highest quality.
| Uncoated lens | Carl Zeiss T* lens | ||
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* Titles and profiles are from the time of the interview.
In August 2005, I was a member of a small group of visitors to Carl Zeiss AG headquarters in Oberkochen, Germany. Since we were about to release top-performance lenses created in cooperation with Carl Zeiss AG for use with α-series DSLRs to the world, I wanted to visit the Carl Zeiss company at least once.
I was most interested in the assembly process for SLR lenses. The atmosphere as accomplished "Meisters" assembled each lens almost completely by hand was intense. The total dedication to ensuring the highest possible quality in each and every lens was tangible, and the experience reconfirmed the fact that Sony would be pursuing uncompromised quality in interchangeable lenses as well. But the deepest, most lasting impression of the visit was received during dinner that evening in the form of a question from a Carl Zeiss executive: "Mr. Katsumoto, is Sony prepared to stay with the single-lens reflex camera business for the next 100 years? This business is not simply a matter of selling cameras. The customer must be confident that the α-mount will continue to exist in order for them to invest their money and faith in Sony."
With those words in our minds and hearts, the "α100" was released a year later in July of 2006. Of course Carl Zeiss AG was our vital partner in that endeavor, and it is our intention that the partnership continue and grow along with our customer's trust.
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