The Leica Lens Saga; An Interview With Peter Karbe Page 2
PK: The optical design of the Summilux 35mm f/1.4 ASPH showed that placing an aspherical element behind the aperture (iris diaphragm) enables an effective reduction of oblique spherical aberration. However, that means that you have to “flatten out” the resulting curvature of field by using high refractive index glass in the positive (convex) lenses. The drawback of these types of glass is that they tend to increase chromatic aberration significantly. I had to find a way to minimize the residual color aberration by choosing glass types with low dispersion and/or anomalous partial dispersion on the one side and glass types with high refractive indices for flattening the curvature on the other side.
The use of special varieties of glass with anomalous dispersion has a long tradition at Leica. Leitz Wetzlar developed several of these special glasses and our knowledge and experience in using such glasses is still of great benefit today for designing optical systems at Leica. We know which glass we need to get the better optical performance and what is particularly important, we know how to handle such glasses in production.
SB: Since this lens took over a decade to develop, and many modern lenses are designed in a matter or weeks or months using computer-aided design and electronic ray tracing, how was the design process that resulted in this lens different from the usual design process?
PK: Our principle is that any new optical design should represent a significant advance over the existing version. The original Summilux 50mm f/1.4 enjoyed an excellent reputation and had a production lifetime of more than 30 years. We therefore knew we had to design an honorable successor, one offering significantly better performance. This took time and needed more than just a series of optimization runs on an optical design software program.
Establishing a balance between factors, including performance, size, choice of the appropriate glasses, and production tolerances, required lots of patient development work and numerous experiments and trials. For example, the glass I chose for lens element 3 is of crucial importance in minimizing the secondary color aberration. This glass, formerly made at the Leitz glass laboratory, was for a long time offered by another supplier who had taken over its production. But they had stopped making it, so I had to “encourage” another German glassmaker to literally reinvent this glass type. Today this glass is extremely expensive. Indeed, the material for this lens element alone costs as much as the glass used in all the other lens elements of the Summilux 50mm f/1.4 ASPH! In short, optical design software, as useful as it can be, will not help in choosing the appropriate glass types, especially those used to minimize secondary chromatic aberration.
SB: Most 50mm f/1.4 lenses are designed to deliver very good image quality (that is high resolution and contrast across the field) at wide apertures like f/1.4 and f/2, but did you also consider the quality of the out-of-focus image, that is bokeh, in designing this lens? And what other subjective image characteristics did you seek to optimize?
PK: Yes, one of our optical design targets is highest performance at wide apertures. For the optical designer at Leica this means balanced correction of spherical aberration and a minimum of residual coma—not only as a design target but also as a production target. This entails maintaining the highest quality standards and tightest tolerances for the production process. “Good bokeh” is the essence of these targets for the use of wide aperture lenses. One key element in achieving it: when you stop down to a small aperture, the iris diaphragm should maintain a nearly circular shape.
SB: Were there any setbacks or special difficulties you had to overcome in designing this lens? How did you accomplish this?
PK: One foible of all high aperture lenses is the fact that these lenses can be optimized for one distance only (infinity). To maintain the optical performance at close focusing distances an additional degree of freedom is needed. This was realized by employing a floating element for the design of the Summilux 50mm f/1.4 ASPH. The challenge was to achieve this floating element without enlarging the size of the lens and keeping the well-known, super-smooth “Leica feel” in the focusing mount. This proved to be a nightmare on the production line and we needed several trials to achieve an optimized production process.
SB: What did you learn from this process that will be helpful in designing future Leica lenses?
PK: The APO Summicron 75mm f/2 ASPH is the second step in this process. One Saturday morning, I woke up and remembered that the Summilux 75mm f/1.4 was derived from the Summilux 50mm f/1.4 (designed by Dr. Mandler) and I decided to try the same basic idea with the 50mm Summicron. Thus, the APO Summicron 75mm ASPH was born. Yes, there are still many other new ideas I would like to realize in the future.
Author’s Note: Leica recently announced three new ultra-speed lenses—the 50mm f/0.95 Noctilux-M ASPH, the 21mm f/1.4 Summilux-M ASPH, and the 24mm f/1.4 Summilux-M ASPH. According to Peter Karbe, the basic motivation behind the creation of the 50mm f/0.95 Noctilux-M ASPH was to showcase their technology and set a new standard. Karbe said, “An integral part of the Leica M tradition is its unique capability for available light photography, so super-speed lenses such as these are essential.”
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