[Leica] Fw: lens design philosophies (1)

["Erwin Puts" <imxputs@ision.nl>]

Optical theory and the correction of aberrations is no secret and wellknown
at every design department. Most departments even use the same program! A
typical design sequence consists of fixing the parameters (aperture,
physical dimensions), then creating a design (number of lens elements etc),
then a rough calculation and assessment of the basic aberrations, and then
the big part: the optimization for a certain predefined merit function (to
be translated as image quality or lens performance in practice).
Cosina does this as does Leica or Konica or Contax, to list the major RF
contenders of now.
What then are the guiding differences between Leica and Cosina, the lens
lines of both I have tested fully.
At the start I may mention that no one can do magical tricks and a lens
costing USD 2500 should be different from a lens costing USD 1000, even if
we allow for economies of scale.
Leica lenses are designed for optimum performance at the wider apertures and
this choice has implications for all the rest.
The amount of aberrations is several magnitudes higher at wide apertures
than at moderate ones. To control these errors one needs to use special
glass, and to adhere to very small tolerances of manufacture and mounting,
and to use material than can stand all kinds of temperature changes. It is
clear that the reduction of optical errors to the micron level implies using
manufacturing techniques to the same level of accuracy. As example one may
note that a slight amount of decentring will show itself with bigger lens
diameters and wider apertures and will go unnoticed in small-diamter glass
at small apertures. And fluctuations in surface roughness will cause
unwanted reflections which will be seen when the glass is very well coated
and extremely transparant, but not so visible when the glass itself is more
opaque.
This state of affairs does indicate that the material costs, manufacturing
costs and labour costs are important parts of the equation when designing a
lens.
The design process is also a factor. Now we need to be somewhat high minded.
A lens consists of let us say 6 elements. We have generally 7 diffferent
aberrations (spherical aberration, coma etc). Some of these lens elements
are more responsible for any one of these aberrations.  When correcting a
lens one could concentrate per lens element on the related aberration or one
can spread the aberration content over all elements. The normal optimization
program throws all aberrations and all elements in one basket and uses
mathematical techniques (Damped Least Squares) to find a minimum for the
sum. Here one reaches a solution quite quickly. Of course one has to assign
a weight to every aberration and lens element for this approach to work and
here the influence of the desiger or team becomes paramount.
Presumably this is the way Cosina works. (most designers do it this way).
Leica has another and more laborious method: they spread the aberration
content as anyone does, but in a different way and they optimize per single
aberration and do a precise balancing of aberrations to fine tune the
residual aberrations. How they do it, is a secret of course, but it takes
much time.
The results that can be reached have to fit the possible level of
manufacturing accuracy and tolerancing.
An aspherical lens can be pressed in a short time or grinded in a day of
work to get at the level of precision required.
End of part 1
Erwin