[rollei_list] Re: Planar, Xenotar, Summicron

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Wed, 25 May 2005 23:17:38 -0700

----- Original Message ----- 
From: "Ardeshir Mehta" <ardeshir@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, May 25, 2005 8:09 PM
Subject: [rollei_list] Re: Planar, Xenotar, Summicron


>
> On 25-May-05, at 7:24 PM, Richard Knoppow wrote:
>
>> [...]
>>
>> In any case, all of these lenses, five element and six 
>> element
>> Planar/Xenotar, and the Leitz Summicron, are all members 
>> of the
>> same family of lenses.
>
> MOST instructive. Thanks.
>
> BTW: does anyone (Richard?) know to which family the 
> (modern) Canon
> EF 50 mm f/1.8 lens belongs? (I'm NOT speaking of the 50 
> mm f/1.4 here.)
>
> (I'd like to know since I have one of those for my EOS 
> Elan 7, and it
> comes highly recommended, optically speaking, even though 
> it's quite
> inexpensive).
>
> Cheers.

   I don't know. If its on a web site somewhere I will look 
at it. However (a big one) Kingslake points out that in 
these days of computer design some modern lenses are 
difficult or impossible to classify as being derived from 
one of the classics. Even some old lenses can be thought of 
as either of two designs or maybe more. For instance, the 
classic aerial survey lens the Zeiss Topogon, designed by 
Robert Richter, four elements all deeply curved meniscus. Is 
it a double Gauss lens or is it a compounded Goerz Hypergon? 
Well, its really both. The Hypergon, for those not familiar 
with it, is an extremely wide angle lens with coverage of 
over 130 degrees, designed in 1900 by Emil von Hough, the 
designer of the Dagor. This lens consists of two very 
steeply curved meniscus elements on either side of a stop. 
The elements are very thin and the outer surfaces nearly 
form a sphere. The lens has a very flat field and very 
little astigmatism, and low coma and distortion due to its 
symmetry. However it is not corrected for spherical or 
chromatic aberration so can be operated only at very small 
stops, less than f/20. The fall of of illumination is even 
more than the rule of thumb cos^4 theta so the lens was 
equipped with a spinning obstructive stop to even it out. 
The Topogon has four elements, the outer ones positive thin 
meniscus as in the Hypergon but the inner ones are negative 
meniscus lenses as in a double Gauss type. The additional 
elements allow it to be corrected for spherical and 
chromatic aberration. The Bausch & Lomb Metrogon has an 
additional element which further corrects the spherical. 
What kind of lenses are the Topogon and Metrogon? As above 
they can be thought of as either double meniscus or as 
double Gauss lenses.
   There are more difficult cases in some modern lenses, for 
instance, few zoom lenses can not be classified as being 
derived from any of the older types, they are just their own 
thing.
   What is interesting is to learn how the various 
aberrations are corrected in the different types and what 
tricks the designers found to correct them. For instance, 
one trick used by Bertele in the Ernostar and Sonnar was to 
use thick, low index, sections instead of air spaces. The 
advantage of this was the elimination of flare while 
retaining some of the benifits of the air space. Paul Rudolf 
found a way of using a cemented interface to vary the 
dispersion of the cemented pair virtually at will without 
having any effect on other optical characteristics. He used 
this trick, called a "buried surface" in the original Planar 
to get the effect of a glass type which was not obtainable. 
Bertele uses the same trick in the f/1.5 Sonnar. Another 
trick, already mentioned, is the splitting of a strong 
element up into two or more weaker elements. Simply 
splitting them reduces some aberrations which is helpful 
when the angles of incidence in the lens become large as in 
very fast lenses or wide angle lenses. Because most of these 
tricks can be adapted to any design they are not really a 
basis for classifying a lens even though the trick may have 
originated with a particular type or be characteristic of 
it.
  Computer analysis of designs has made a huge difference in 
design technique. The method of evaluating a design is the 
trace rays of light through it. About three rays are 
necessary to get any idea of what its doing. By hand methods 
a single ray trace will take perhaps half an hour. If a hand 
calculator is used this can be reduced to perhaps five 
minutes. Any of the common computer optical design programs 
(OSLO, Zemax, etc.,) operating on a fast PC, can make 
millions of tray traces in a fraction of a second. Its 
possible to get a very complete analysis of a prospective 
design very quickly and to derive presentations of the 
information which were not practically possible before 
computers. The ray tracing is so fast that the computer 
program can be set to vary certain parameters to optimise 
the design, but as Kingslake and Warren Smith point out the 
program can't always tell when it is getting into 
impractical areas so it needs human guidance.
   This is not to say that all old designs were less than 
optimum. Brian Caldwell, a well known lens designer and the 
author of the program LensVIEW, says than many of the old 
Zeiss designs are so close to optimum that computer 
optimisation, even with changes in glass to modern glass, 
does not improve them significantly. This is partly due to 
very careful calculation but also because the old method of 
design was to evaluate the presciption mathematically until 
it looked pretty close and then build a model of it. 
Optimisation was then done by poking at the actual lens 
until it performed as well as could be gotten.
   Some advantages of modern design are less significant 
than might be thought. For instance aspherical surfaces have 
been around for a long time. Zeiss used them in some 
experimental lenses included in the survey of lenses called 
the Zeiss Index. An asphere can be duplicated by several 
spherical surfaces. The advantage of the asphere is 
simplification of the lens. Modern manufacturing processes 
allow economical production of aspheres. In the past each 
one had to be hand figured.
  Another advantage of the last sixty years has been the 
avialability of glass with very high indices of refraction 
and relativly low dispersion, or low index-high dispersion 
glass. All glass bends light. The amount it bends is related 
to the Index of Refraction. The idex is the bending of light 
compared to a vacuum, which has an idex of 1. Actually air 
is so close to 1 than it is usually considered to have an 
index of 1 except for the most precise work. The index of 
refraction is also the ratio of the speed of light in the 
medium to the speed in a vacuum.
   Now, things would be fine if the value of the index of 
refraction a constant. It isn't: it varies with wavelength, 
generaly going up as the wavelength decreases. The effect is 
known as dispersion.  This is why a prism splits white light 
into a spectrum. The same effect is produced by simple 
lenses. In fact, two prisms, base to base, are an elementary 
positive lens. This effect is known as chromatic aberration. 
It is corrected by combining a positive element with a given 
amount of dispersion with a weaker negative element with 
abuot the same amount of dispersion. If the dispersions are 
nearly the same they will cancel. However, in order for this 
conbination to have any power the positive element (assuming 
we want a achromatic positive lens) must have more power 
than the negative lens. Practically, this means combining a 
positive lens with a high index but relatively low 
dispersion with negative lens with lower index but the same 
dispersion. Before Abbey and Schott came up with the Barium 
glasses known as Jena glass, in the late nineteenth century, 
all glasses followed a line where increasing index was 
accompanied by increasing dispersion. This meant that 
positive and negative lenses had to be assembled in a 
certain way to cancel the chromatic aberration. The 
disadvantage of this is that the reverse combination of 
positive and negative was needed to correct astigmatism. So 
that it was impossible (they thought) to make a lens of old 
glass that was both chromatically correct and free of 
astimatism. As it turned out, this was not true but a 
chromatically correct anastigmat of old glass was not 
produced until the 1920's by K. Martin of Busch (the Omnar). 
Even so the invention of the new type glasses furthered 
optical design enormously. The range of glasses was 
increased even more by the development of rare-earth glasses 
at the United States National Burea of Standards beginning 
in the early 1930's and developed by Eastman Kodak in the 
late 1930's.
   By increasing the index of a glass, especially if the 
dispersion can be kept low, the curvature of the surfaces 
for a given amount of power can be reduced. Since several 
aberrations are proportional in some way to the angle of 
incidence of light at the lens surfaces the glass 
automatically reduces the aberrations. This allow either an 
improvement in performance with a given amount of complexity 
or a duplication of performance with a simpler system.

I've written too much.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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