This issue, a look at the intriguing design of a new Voigtländer 50mm f/1.0 lens
Recently my eye has been caught by the appearance of a new lens, which seems to be quite a departure from the norm. The lens is the Voigtländer 50mm f/1.0 Nokton Aspherical, to give it it’s full title. Optically the design of this lens follows wellestablished patterns for such a fast lens. The front of the lens is a six-element double-gauss design, which could have been found in any fast normal lens since the 1950s – with one exception, of which we will talk later. Built into the rear of the lens is what has become misleadingly known as a ‘speed booster’ with three elements in two groups. The front lens will have been designed with a longer focal length than the 50mm marked on the lens while the back part reduces that focal length to 50mm thereby reducing the f-number to f/1.0. This is a similar design to the 1966 Zeiss Planar 50mm f/0.7 famously used by Stanley Kubrick in his film Barry Lyndon, though not as ambitious, at least so far as the f-number is concerned.
So, the basic configuration is not novel. It is the front element that is notable, as its front surface is aspherical. But the usual manufacturing processes for aspheric lens surfaces are not suitable for use in this exposed location in a lens.
Aspheric surfaces are a lens designer’s dream. At a stroke they can overcome spherical aberrations that can take many traditional spherical surfaces to correct. However, manufacturing them in quantity has always been very expensive, until recently.
Two processes have been developed to overcome this problem. The first is ‘hybrid aspheres’. Here a resin (plastic) veneer is moulded with one face spherical, to match a glass lens, and the other aspherical. When this veneer is bonded to a glass lens the result is a lens with an aspherical surface.
The second technique is precision glass moulding, where a glass lens is pressed from an exact mould with the required surfaces. Both these techniques have problems for low volume lenses, given that the set-up costs for the precision mould is very high. In terms of their utility as a front surface, the problem is the material used. Neither optical resin, nor the special glasses necessary for precision moulding, have the durability required. Hence the Voigtländer lens’s front element is ground and polished just like a conventional spherical surface.
This production method has in the past been so prohibitively expensive that it has been restricted to
