[Leica] Summicron 28 part 1

[imx <imxputs@knoware.nl>]

Introduction and background.
A few years ago in Solms, I was shown a  lens, a compact 1:2.0/28mm lens for
M. It was a prototype, to be sure, but it was ready for production, all
optical computations and tests were done. On my question why this lens never
made it to the production stage, Mr. Kölsch answered, that he expected more
performance from a new lens of these specifications. So a new design was
created from scratch: the Summicron-M1:2/28mm ASPH., scheduled for delivery
early in 2001.  
The optical prescription of the lens is quite fascinating. It fits in the
genealogy of the seminal Summilux ASPH, a design that decisively departs
from the classical Double-Gauss formula. This design-type, now more than a
100 years old, has been stretched to the limits and a performance plateau
has been reached.  The new Summilux design, incorporates the negative front
and back surfaces and the aspherical surface. It is probably the first lens
that has been designed specifically around the use of aspherics. Retrofocus
designs are a second approach to step out of the shadows of the Double-Gauss
formula. More lens elements can potentially improve performance, as more
parameters can be controlled. The new Summicron-M 1:2/28mm ASPH picks up
design elements of both: the lens group in front of the aperture is an
enhancement of the Summilux (front group) design and the lens group behind
the aperture fits into the retrofocus family and is a derivative of the
2.8/28 formula. We should not press the point, however, as a lens design is
a creative whole and not a mix of ready made components. The message should
be that the new Summicron is based on the best design principles currently
available in Solms thinking. The location of the aspherical surface is
different and probably decisive for this design.
The ergonomics.
The new 2/28 is indeed a very compact lens, comparable to the current 2.8/28
version. 
Measurements are (2.8 version in parentheses): length from flange: 41mm
(41.4mm), overall diameter: 53mm (53mm), front diameter: 49mm (48mm). Both
lenses use filtersize E46.
For a lens with twice the speed this is a remarkable feat. This design
indicates the direction of future Leica M designs: compact and high speed
and high performance. The somewhat weak performance of the old Summilux
1.4/35mm could be excused with reference to its compactness, which forced
the designers in those days to find a compromise between size and
performance. Now the circle has been squared.
The lens operates very smoothly, and the aperture ring clicks with just the
right amount of resistance and fluidity. When taking pictures with the new
Summicron 28, I was amazed how quickly I could focus  with the focusing tab
and I have to confess that I hardly missed a shot, when focusing moving
objects. The depth of field with a 28mm lens, even at an aperture of 1:2
exceeds of course the DoF of the Summicron 50 by a factor of 2, which brings
real advantages in street shooting.