[rollei_list] Re: Planar, Xenotar, Summicron

  • From: Carlos Manuel Freaza <cmfreaza@xxxxxxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Thu, 26 May 2005 10:59:35 -0300 (ART)

Richard, it could be an article for a magazin, very
interesting, the reference about the old and new means
to design lenses specially, thanks.

All the best
Carlos




 --- Richard Knoppow <dickburk@xxxxxxxxxxxxx>
escribió:

> 
> ----- 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.
> 
=== message truncated ===



        

        
                
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