Through its special optical design, the new family of high-end SLR lenses exploits the full potential of high-definition sensors.
At the end of 2013, the Distagon T* 1.4/55, the first member of the new high-end SLR family of lenses will come on the market. The most important goal in developing this lens was to achieve a “no-compromises image quality”. Which of the optical design characteristics achieve this? Dr. Aurelian Dodoc, ZEISS Principal Scientist, explains.
ZEISS has announced a lens with “no-compromises image quality”. What exactly does that mean?
In the history of camera lenses, you’ll find many lenses whose quality could be described as good. But our goal this time was to create perfect lenses for those who understand and value the difference between good and perfect. Compared to our previous SLR lenses, which already had a very impressive quality, we have now raised the bar much higher. The result is a family of high-quality lenses with perfect imaging qualities.
The new lens offers optimal imaging performance, even with an open aperture. What did you need to do in the optical design to achieve that?
We decided to use the complex Distagon optical design, which until now has only been found in wide-angle lenses. This is the only way to get the desired sharpness and rendering of contrasts all the way into the corners of the image.
Why is the Distagon better suited for sharpness and contrasts than another design concept?
Traditional lenses in the focal-length range of around 50 mm are of the Planar type. With this lens type, with its practically symmetric, very compact construction, there is a close correlation between the correction in the center of the image and at the edge of the image: when there is good correction at the center of the image center, the edges are not corrected, and vice versa. Expressed more precisely in technical terms, the correction of the spherical aberration does not correct the image field curvature at the same time. Our most important goal in developing this lens was consistent imaging performance across the entire image field, including at the edges. Which is why we replaced the Planar’s relatively simple structure with the much more complex structure of the Distagon.
The Distagon allows spherical aberration and image field curvature to be corrected at the same time. Can you elaborate on that in more detail? As an optical designer, how did you accomplish this and how does it manifest itself in the image quality?
The structure of the Distagon allows for a good split between the negative and positive optical refraction power within the lens. As a result, we were able to achieve an excellent correction of the image field curvature. The image field curvature is the most important image defect in photography, and correcting that is a prerequisite for getting a perfect image. With an adjusted framework of lens elements and various aspherical surfaces, the other aberrations can also be corrected to the minutest levels. In particular, we have perfectly corrected the spherical aberration and the coma, both of which can have a devastating effect on the quality of an image. Distortion is also imperceptible.
In addition to bringing consistent sharpness into the edges of the image, it was also important to us that the edges of the object appear as natural as possible. This required extreme color-error correction and the prevention of color fringing, which can be caused by chromatic aberration. The Planar system would not be able to handle this: since the lens elements in the Planar that take on the role of this correction are also responsible for the correction of the spherical aberration and for the image field curvature, it is very difficult to correct color defects when using a Planar. The only way to solve this challenge was to use the Distagon. With ‘normal’ optics, a good chromatic correction in a high-quality optical system is simply not possible. To correct color defects, we therefore use special lenses that have anomalous partial dispersion. This is how we corrected not only the primary and secondary color defects, but also the chromatic variations of all aberrations, which also strongly reduce color fringing.
Another important feature of high-quality lenses is a lack of stray light. How did you avoid stray light outside the angular field?
Stray light, or false light, is caused by reflections of all kinds, but also by the reflections between the optical surfaces. For this we used newly-developed multiple layers. ZEISS is also a leader in this field of technology.
In addition to the Distagon and multilayer design, the lens’s inner focusing is another unusual technical feature when it comes to this focal length. Why did you decide on this type of construction?
With inner focusing, focusing does not happen by shifting the entire lens, but rather through one or more lens elements within the lens. We have built them so as to maintain the performance of the lens for even the smallest distances of 350mm to the object. The lens elements after lens 4 move in the direction of the object. As a result, the actual length of the lens does not change during focusing.
Many photographers are used to taking pictures with autofocus these days. Why does the new lens only have manual focusing?
The lens is probably the most powerful standard focal length in existence and we wanted to enable photographers to take full advantage of its performance. We want the user to have the freedom to employ the focusing position as an artistic means, since we are dealing here with one of the most important tools that a photographer has at his disposal. Moreover, the lens offers very fine haptics and an extremely precise focusing mechanism. The lenses of this new family are therefore suited above all to application areas in which conscious, manual photography shows its true strengths, such as portraiture, fashion or landscape photography.
What do photographers like about a lens in this performance class? And what do you think photographers will do with this lens?
With this new family of lenses we want to offer photographers an artistic tool with which they can create perfect pictures. The optics should not pose any limits on the quality of the image. A photographer’s creativity should be the defining factor. We want to challenge photographers to create the perfect picture by providing the perfect optics.
For a more detailed insight into the physical and technical characteristics of lenses we recommend reading some articles from our archive: