[rollei_list] Re: Planar, Xenotar, Summicron

  • From: Ardeshir Mehta <ardeshir@xxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Thu, 26 May 2005 12:44:56 -0400


Wow. This one's a keeper for the archives! Thanks again Richard.


+++++


On 26-May-05, at 2:17 AM, Richard Knoppow wrote:

> [...] 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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