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 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 no utilize any assembly lines.
Instead, all production and assembly steps are done on individual work
stations, usually 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
with a 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 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. 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 able 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
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 this method 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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