Getting the right balance for the perfect image
Whether you are taking extraordinary aerial photographs, atmospheric panoramic shots or captivating images of the polar lights, you can rest assured that Carl Zeiss lenses will deliver consistently superb quality and outstanding results even under challenging conditions. Christian Beder, an optical designer at Carl Zeiss, explains what makes the lenses so special.
What is the biggest challenge in producing lenses?
It all comes down to finding the perfect balance. In optical design, we dictate how the optical system is constructed, specifying everything from the number, size and form of the optical elements to the air spaces and coating. To do this, we have to take numerous issues into account, not only image quality aspects such as sharp focus and freedom from distortion, but also mechanical limitations and budget constraints. Generally speaking, you cannot simultaneously satisfy every single requirement. If the main emphasis is on price, then it is, of course, possible to manufacture an economical lens – for example by reducing the number of optical elements – but then you have to be willing to accept inferior image quality, which is not something we are prepared to do. But if you focus solely on imaging performance, then the costs quickly get out of hand. We endeavor to strike just the right balance to ensure that our lenses offer the best possible combination of image quality, price, size and weight.
What are the key factors you have to take into account?
The ultimate goal is that the lens should produce outstanding pictures: razor-sharp, distortion-free images – without any chromatic aberrations – that stay crisp and bright right to the edges. Not only does that require in-depth knowledge of the technical parameters, it also requires the ability to handle those parameters creatively. Optical design at Carl Zeiss is based on 120 years of in-house experience, but it also draws on tremendous sensitivity and the creative urge to tap into new possibilities.
What would it be like if you could work without any of these constraints?
Then there would be very little artistry involved in developing the perfect lens. It would be about one meter long with a diameter of 50 centimeters and would cost millions, but it would be as perfect as the laws of physics allow!
Wide-angle lenses pose a particular challenge to optical designers. Why is that?
Well, their wider angle of view means that they show a large portion of the subject. In this situation it is quite a challenge to produce a consistently good image of the entire field of view. There are three field-dependent image errors that are particularly significant in this context: distortion, field curvature and astigmatism, a form of aberration that causes the vertical and horizontal lines of the image to appear at varying levels of quality.
There are different types and forms of optical elements that can be used to correct these aberrations. The most important aspect is ensuring that the lens elements are correctly positioned, which means they need to be placed a long way from the aperture – generally at the front, which is where they can effectively counteract this problem. That is why so many wide-angle lenses have a tapering shape that makes them bigger and heavier at the front.
Why is it that ZEISS lenses tend to be more expensive than lenses made by other manufacturers?
That stems from two factors that lie behind the high quality of the lenses, namely low tolerances and an almost pedantic obsession with quality control. On its journey from the raw material phase to the finished product, each lens passes through more than 50 quality control stages.
None of our lenses can leave the production hall unless they have been thoroughly tested, and the same applies to components manufactured by our partners. And, on top of that, we demand extraordinary precision in the manufacture and assembly of our lens elements. The surface of a lens element is never entirely flat; the way it is constructed means that it will inevitably feature certain irregularities. By having lower tolerances for this surface unevenness we improve the image quality of the lens but simultaneously incur more costs in production. We set some of our tolerances as low as just a few hundred nanometers.
What is the greatest achievement ever in the field of optical design?
I would have to say the development of the anti-reflective coating, which Carl Zeiss subsequently developed into the multi-layer coating T*. This coating is applied in a vacuum by means of vapor deposition and serves to reduce the amount of light reflected from the surface of the lens, so it essentially improves light transmission and suppresses unwanted reflections. It is only thanks to this invention that we can now make optical systems with more than five or six lens elements – without this coating far too much light would be lost through reflections. Some of the zoom lenses Carl Zeiss currently makes for cinematography applica-tions have more than 25 optical elements. The more elements an optical system has, the better the image quality – but only if we are able to adequately reduce the amount of reflection, which is precisely where the T* coating has played such an essential role. This does not apply in a completely linear fashion of course – doubling the number of lens elements does not necessarily lead to a doubling of image quality – but there is certainly a clear correlation. It works because the individual lenses are able to cancel out each other’s optical aberrations.
What are the latest trends in optical design?
Reducing chromatic aberrations is becoming increasingly significant. Digitalization means that anyone can now view their images in unprecedented sizes. Nowadays we look at an image on the screen that corresponds to a picture measuring 2×3 meters from a distance of 30 centimeters, which in some ways provides an absurd, unnaturally enlarged view. It enables users to detect chromatic aberrations on an individual pixel level that they would never have seen before. So our job now is to correct these aberrations by using more and more lens elements and increasingly specialist and more sophisticated types of glass. So digitalization is also having an effect on lenses!
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