Leica 50mm f/2 APO
Summicron M ASPH
All of the major lens manufacturers are capable of making lenses of the highest possible quality, yet when taking overall lens performance into consideration, Leica lenses are the ones that usually stand out. That of course brings up the question “why?” The main reason generally given is that Leica is the only major lens manufacturer that does not mass produce any of their lenses. Mass production, as good as it has become, is simply incapable to generate the overall quality that Leica can produce with their approach.
If it were possible to make a perfect lens, one without any flaws, this article would be meaningless. However, it is impossible to do so. Each lens will display certain faults which are virtually impossible to overcome and, unfortunately, of these faults or aberrations there are many and all of them will cause the photographic image to deteriorate. It is not my intention to go into the problems of lens design as such; instead I want to talk about what can be done to keep aberrations to a minimum, to make a lens perform as well as possible.
The easy answer is obvious; keep aberrations to a minimum, eliminate them as much as possible. Every lens designer and manufacturer does this, within the parameters given to them.
The answer is actually quite easy, but very difficult to realize. The key word is tolerances or precision. The more precise a lens is designed and made, the closer it comes to an ideal, flawless lens, and the better the lens will perform.
During my visits to Leica I was able to obtain a wealth of information about what they are doing to make lenses as good as possible. That amounts to tolerances and a level of precision simply not available from other manufacturers. This takes place on two levels, the mechanical and optical precision. For reasons of greater accuracy, Leica does not utilize any assembly lines. Instead, all production and assembly steps are done on individual workstations, in clean rooms with the work station functioning by design as additional clean environments.
All production and assembly steps are done at individual work stations
The mechanical tolerances applied by Leica must conform to a minimum of 1/100 mm or 10 micrometers which equals less than 0.0004 inch for the accuracy of the lens mounts of both cameras and lenses, but also for the accuracy of the focusing mount of their lenses and for the rangefinder in the Leica M-type cameras. Other manufacturers apply tolerance of about half of these measurements. The reason for this is that the performance of several Leica lenses would be compromised if these tolerances would not be adhered to. For the focusing mounts of all manual lenses, initial testing is done by machine. But the final test is up to an experienced individual who is trained to feel even the slightest imperfections in the smooth operation of the mount. No machine is capable of doing that.
Work on lens and focusing mounts being performed
Adjustments for focusing mount being performed
The optical tolerances of Leica lenses go far beyond this. This starts with the various types of glass being used to make a lens. Leica used to make their optical glass in house, but that proved to be too expensive in more recent years. Instead Leica now purchases their glass from companies like Schott and Hoya. This includes proprietary designs like the so-called Apo glass which was first developed by Leica for the 50mm f/1 Noctilux.
This Leica 900403 glass consists of no fewer than a dozen different ingredients, including the rare earth element lanthanum. One kilogram of this glass costs almost 60 times as much as a common optical glass such as BK7. Please keep in mind that this APO glass is twice as heavy as BK7. Subsequently the volume of glass in a kilogram that can be turned into lens elements is only half as great, making the actual price ratio between these two optical glasses approximately 120-to-1
In many cases the factory receives the glass as pressings which roughly approximate the final shape of the lens. Other glasses can only be obtained in blocks which must be sliced into sheets, cut into squares, cemented together and ground to from a stack of discs. These work pieces are then cemented onto the appropriate grinding and polishing heads. About 100,000 of these grinding heads must be on hand. Until a few years ago, the grinding heads were made large enough to grind several lens elements simultaneously. However, for reasons of greater precision, Leica grinds each lens element individually in modern machines which, in most cases, keep the glass stationary while the grinding head rotates. Each grinding step is immediately accompanied by a check for proper tolerances before the element is passed on to the next step.
Lens Production - Production of Asphericals - Grinding
Various lens grinding machines
Tolerances differ substantially already with the raw glass. Leica applies a standard of ±0.0002% for the accuracy of the refractive index. This compares to the international standard of ±0.001% as applied by other lens manufacturers. The accuracy of the Abbe number, the measure for dispersion, is ±0.2% for Leica compared to ±0.8% internationally.
Once the raw glass blanks have been received and tested for the proper accuracy of their properties, they are ground to their specified shapes. For the manufacture of individual lens elements Leica allows production tolerance of no more than ¼ lambda or ¼ of the average wavelength of light which corresponds to approximately 500 nanometer or 0.0005mm for the accuracy of the lens surface. In comparison, the tolerances applied by other lens manufacturers are ½ lambda or 0.001mm. Similar tolerances are used for the thickness of the elements and proper centering along the optical axis.
As of late many manufacturers are offering lenses with aspherical surfaces which can greatly improve lens performance by virtually eliminating spherical aberration. However, there are two distinctively different approaches in the manufacture of these elements. An inexpensive method is to produce a “conventional” spherical element and sandwich it with a thin aspherical surface element. These are made of precision molded acrylic. However, this method, originally developed by Zeiss, was ultimately discarded by them because it could not approach their quality standards. The main cause was the fact that even the clearest plastics, like acrylics, consist of very large molecules. Light, when transmitting, literally will scatter off these molecules, causing the light to be slightly diffused, which ultimately has adverse effects on lens performance. Other companies use precision molding equipment where a glass blank is reheated until it becomes pliable and then is precision molded into the final shape of the lens element. Some exotic types of glasses cannot be used with this method because the reheating and molding will cause the glass to deteriorate and thus make it useless. Subsequently, this precision molding process forces the lens designer to compromise to a certain degree because better suited glasses cannot be utilized. The same applies to lens elements of larger diameters. Leica uses an entirely different approach. They use computer controlled automatic grinding and polishing of the glass elements which require the adherence to extremely tight tolerances. Unfortunately such production methods can only be achieved at considerable expense.
Measurement for accurate thickness of the lens element
Each manufacturing and assembly step is immediately followed by a check
For the production of aspherical lens elements Leica applies tolerances which cannot exceed 0.03 micrometer or 0.00003mm. To achieve such precision Leica employs special grinding machines where the lens element is rotating against the grinding head, which is in form of a narrow rod. This will grind only a small section of the entire surface of the lens element at a time. A special grinding substance is also used which is partially magnetized. This is done to allow for a more precise adherence of the grinding substance to the lens and grinding rod surface. With all lens elements the grinding substance becomes ever finer from one step to the next until it is mostly water with a small amount of a very fine polishing compound. Unfortunately I was not permitted to take specific photographs of the manufacture of aspherical lens elements.
The grinding compound for many lens elements must be continuously rotated to avoid deterioration
Unlike with other manufacturers, at Leica all individual lens elements, spherical and aspherical, do not approach their final surface configuration and thickness until the lens coating is applied. Lens coating at Leica is not applied in the same manner for all elements. Instead selective coating layers of different substances is applied in a manner that eliminates surface reflection as much as possible.
Grinding aspherical surfaces via the method used at Leica is extremely time consuming. As with all manufacturing steps at Leica, each individual step is immediately followed by a check. If these checks show that deviations from the norm still exist, the step is either followed by additional work, or the lens element is discarded. This often leads to no more than five aspherical lens elements being produced in a single day.
To increase lens production, Leica tried to outsource the manufacture of some aspherical lens elements to other companies like Schneider Kreuznach, for instance. Unfortunately this proved to be a dead end. The companies that were approached by Leica either were not able to work within the specified tolerances or they simply were not able to supply a sufficient number of elements to make such cooperation feasible.
Applying paint to the edges of the lens elements
Before lenses are assembled, each lens element is coated at the sides with black paint to avoid reflections. This used to be done with a brush, but now a specially designed foam applicator is used instead. The question is often asked why this isn't done by machine. The simple answer is higher accuracy. The general black coating is easily applied with just one step. However the often sharp edge between the polished lens surface and the edge often requires as many of five additional applications to be perfect. This can only be done by hand.
All of this makes Leica by far the foremost and most advanced manufacturer of aspherical lens elements in the world.
There are a couple of additional steps in the manufacture of Leica lenses which are virtually ignored by other manufacturers. It is a known fact that regardless of how precise one tries to make each individual lens, there will always be slight fluctuation in performance from one lens to another. To minimize these fluctuations. Leitz tests each individual lens element after the completion of all manufacturing steps to determine its actual focal length and along with it the fluctuations from the ideal. If these do not fall within the rather stringent parameters set by Leica, these elements are usually discarded. The other ones are described with plus or minus figures to indicate the deviations. Then, during assembly of the optical components, these figures are used to even out the differences with the result that there are considerably less fluctuation in performance from one lens to another.
Finally, there are some additional steps when it comes to the Leica rangefinder lenses. In order to work with utmost focusing accuracy in conjunction with the mechanical Leica rangefinder, the actual focal length of each lens is measured very accurately. Older Leica lenses were even engraved with these figures. For instance, my 90mm f/2.8 Elmarit has the figure 05 engraved to the right of the infinity mark on the focusing scale while my 135mm f/2.8 elmarit reads 55. If my memory serves me correctly, this means that the 90mm is actually a 89.5mm lens while the 135mm in reality is a 135.5mm lens. The focusing cam of all Leica rangefinder lenses is then ground by hand to reflect the actual focal length of the lens to make rangefinder focusing as accurate as possible. Unfortunately the extra precision that goes into every Leica lens is also quite expensive. But it proves once again, you get what you pay for.
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Doesn't designing lenses with a computer pretty much take care of any possible differences in lenses?
ReplyDeleteNot at all. Lens design is a science that has developed over a large number of years, actually predating photography. A computer can only work with the programming that it has been subjected to, subsequently experience does play a factor here from the very onset. A computer design can lay out the specifications of a lens, however, the execution of that design and the adherence to tolerances is something that depends on the lens manufacturer and on the parameters which they set for themselves. This starts with the decision of mass production vs. a bench-made process as used by Leica. The ultimate performance of a lens is very much influenced by how small a tolerance level is considered adequate. Mass production will never approach the overall small tolerance level utilized by Leica and it is that what ultimately sets them apart.
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