PERIMETAR,  SPHAEROGON,  PLEON:

THE  DEFINITIVE  COMPENDIUM  ABOUT  THESE

SUPER-WIDE  AND  FISHEYE  LENSES  OF  THE  '30s

CONCEIVED  BY  THE  FIRMA  CARL  ZEISS  JENA

(10/03/2010)

Some years ago I wrote a similar article (in Italian), published on this website, but further documentations and evidences convinced me to remaster the whole stuff and realize a new one, more detailed and provided with unpublished pictures and drafts; hope you'll like it.

The firma Carl Zeiss Jena was also nicknamed "dream's catcher", and this appellative perfectly frames the situation if we consider the valiant effort squandered in mid- and late '30s to materialize new and intriguing visual perspectives: ingenious projects called Topogon,
Perimetar, Sphaerogon and Pleon opened the third eye towards unexpored worlds, even if - honestly - they where more in tune with an imminent war effort than with creative souls.

Perimetar, it's name just explains, was a wideangle lens defined in 1935 by Willi Mertè, a lens
able to spread a square angle, reduced on the effective film format to a more conservative 80°
@ f/6,3, an anyhow impressive figure for a mid-30s product and a step forwards upon
the former Tessar 2,8cm f/8 of the Contax outfit, limited to 76° with a modest maximum aperture; the Perimetar was a completely new lens design, with priority to air-to-glass  surfaces' number reduction and the choice of a concave front lens that recalls some modern
Leica M aspherical lenses; the profusion of small glued triplets with difficult to master Fluor Krown low dispersion glasses foretell a costly and careful production.

Interestingly, the limitation of air-to-glass surfaces let us guess that the Perimetar was not intended for coating, backdating the first Mertè's computations of this lens months or years before the firsts versuche (1935), when Smakula's coating vacuum process was discovered at Zeiss.

The aforementioned complex and expensive framework hindered a serial production, even if the already assigned brand name betrays the intention to commercialize the Perimetar; actually are known seven different models: six versuche and a 25mm f/6,3 bearing the brand name, intended as a pre-production trial for the definitive Contax lens version. The first versuch is labeled as a 100mm lens and the only exemplar actually known is the pre-production 25mm f/6,3 with serial number 1.503.342; uncording to Hartmut Thiele, this lens belongs to a batch of 50 serial numbers (between 1.503.301 and 1.503.350) planned for pre-production, but even if it should be the 41st exemplar of the row, no other exemplar apparently survived to wartime (or maybe it's waiting somewhere for a cameras' Indiana Jones...).

 

These drawings show two embodiments in the original Perimetar patent by Willi Mertè; the sections confirm the care for air-to-glass surfaces reduction, following the mainstream of other lenses like Dagor and Herar; Willi Mertè was a brave fan of this concept, and in the early days he had already computed achromat lenses with four elements in a single group! The Fluor Krown glasses are located respectively in second and third position in the two cemented triplets, and maybe, if produced, the lens would have reported separation problems due to the particular thermal expansion and poor adherence of these materials. The second Fluor Krown lens of the front triplet, in both embodiments, is glued to Dense Krown glasses of identical specifications. The second exemplar, with a further thick lens behind the stop, should seem the more advanced option, but analyzing the drawings of every prototype we must infer that the evolutionary process walked backwards, and the latter ones show the simpler configuration with six lenses glued in two triplets, a forced, budget-saving choice that lead to further overtime to correct a visible distorsion (while a third computation not included in this patent and spread over nine lenses assured a barrel distorsion kept around the 1% figure, an excellent result for an unsymmetrical 80° wideangle of this era). The concave front surface is the most distinctive feature of this project.

This is the only exemplar of pre-production Perimetar 25mm f/6,3 actually known; it bears the 1.503.342 serial number and belonged to the huge Zeiss lenses collection picked-up in Jena
by the U.S. forces and shipped to the Signal Corps Engineering Laboratories of Signal Corps'
Army Pictorial Division, lenses scheduled by Edward K. Kaprelian and Willi Mertè himself, moved to the U.S.A. (under the aegis of operation Paperclip) where he died shortly after.

 

These drawings show the three Perimetar formulas: the more complex was adopted for the pre-production 25mm f/6,3 n° 1.503.342 and for early versuche drawed in 1935; the last
1935 Perimetar versuch (V1935 n° 16B) is the only one equipped with the seven lenses formula (the second embodiment of the original patent), where the back element consists of a single, thick lens; unbelievably, the more advanced prototypes computed in 1936 and 1937
came back to the simpler option, a six elements within two cemented groups.

 

Here you are the three options with more details; the first, simpler formula bears the stop behind the lenses' groups, in unsymmetrical position, and this unlucky place leads to some distorsion, even if the stop was located just a shade behind the last lens (the iris - in a 25mm
lens - slides at a mere 0,79mm - 0,0311" from the last lens vertex, dusting off the lens' apexes). Uncording to Merte's claims, the Perimetar was intended not only for taking conventional pictures but also for reproduction in a scale exceeding the 1:1 ratio (in reverse position); he fount a satisfactory balance arranging a front (glued triplet) member having a stronger refractive power than the second, and a better correction was achieved with a second triplet where the refractive power of the single lens elements shrinked along the front-back direction, while  the relative dispersion of the glasses followed the same path; the
 good overall correction of lenghtwise sperical aberration allowed a maximum aperture of f/6, retouched in most exemplars to a more conventional f/6,3. Flare and reflections are well restrained, thanks to the limited air-to-glass shifts, the position of the iris and to the concave front element. In the third option the back member was changed to a third glued triplet, and the refractive/dispersive index path is the same as in the second triplet, with a similar glasses' choise: maybe this sophisticated third member allowed a more refined correction of distorsion and other aberration, but was far too complex for a mass production with the '30s tooling tolerances. Referring to actual Abbe's tables, are still recognizable some glasses produced right to our era, like the Barium Flint BaF10, the very light Flint LLF1 and the Fluor Krown FK5 (at time called FK1).

During the years straight after the war, Willi Mertè drawed up the technical sheets of the lenses included in the Zeiss collection, just before his death occurred in Dayton, Ohio, in 1948; these index-cards, burdened with an incredible historic and technical value, was reproduced on roll films and kept in store inside the Air Technical Index data collection by the Central Air Document Office, at the Wright-Patterson Air Force base always located in Dayton (Ohio), under the reference codes REEL-C 2960B - A.T.I. 38399; unfortunately, during more than 60 years, these roll films deteriorated, and actually the copies of the original sheets, for my greatest regret,  are partially illegible. The following images are reproduction of the sheets related to the various Perimetar versuche.

 

The first versuch in chronological order is the V1935 n° 2 and it's the only one whose focal lenght differs from the 2,5cm - 25mm (1,016") figure taken as standard for the other options;
his rechnung was dated 18/03/1935 and, uncording to Thiele, the serial number of the prototype was 1.674.885. The focal lenght of this lens was 10cm - 100mm (3,937") with an aperture of f/6,2; the distorsion is barrel-shaped and kept within the 2% mark. As you can see, this early prototype shows the more complex optical formula with nine lenses in three groups, a configuration omitted in the original patent. The longer focal lenght involves boosted sizes upon the only survived exemplar: the whole lenght of the optical sketch is 58,5mm (2,303") and the front lens diameter is about 60mm (2,362").

 

The second prototype, called V1935 n° 16, was fitted with a similar optical nut, but with more room between the second and third glued members; also, the cutting shape of the rear lenses appears different, but maybe it's only a sketch's simplifying; the focal lenght of this lens (the same as in the following examples) was lowered to 2,5cm - 25mm (1,016"), preconizing a service on 24x36mm cameras; the distorsion figure is similar to the previous one, with a barrel shaped profile peaking at a shade under 2% and reducing to near zero in the far corners.

 

The third versuch is the V1935 n° 16B, the only  fitted with the optical formula shared with the second embodiment of the patent; it points out a simplified rear member realized from a single block of dense Krown glass; the distorsion figure in far better, not exceeding the 1% value, with zero point and some "moustache" figure in the extreme corners. Some authors, mistakenly, report this lens as V1936 n° 16B (maybe a misprint).

 

The fourth exemplar of the row is the V1936 n° 6, a 2,5cm - 25mm (1,016") f/6 lens whose optical formula relates to the less sophisticated embodiment of the patent, a six lenses in two glued groups configuration that, at a glance, looks in some way "incomplete"... Whereas, the lens is effective and shares only a small aberrations' increase; the barrel shaped distorsion is only marginally higher, even with a behind-the-lens iris. In the faded sheet it's still possible to recognize the glass codes of the back triplet: BaF10, LLF1, FK3. The serial number of this prototype is reported to be 1.866.137.

 

The fifth embodiment was the V1937 n° 1, a 2,5cm - 25mm (1,016") lens shaped like the previous one; the full aperture was set to f/6,3 and the distorsion figure discloses the meticulous fine-tuning actuated by Mertè and the intention to develop this cheaper formula, crossing the more penalizing aberration related to this particular lens shape. In this embodiment the designer choosed to keep the barrel distorsion near zero on field, with a "moustache" inversion to pincushion towards the borders.

 

The last prototype (V1937 n° 4) follows the same streamline, and the correction concepts of the previous lens was forced, obtaining a distorsion near zero from the axis to half frame, after that quickly changing to pincushion till the corners, where we can read a boring 4% figure.
The rechnung of this lens is dated 15/02/1937 and depicts the last development of the Perimetar concept before renouncing to further efforts: maybe the field angle was too narrow for Luftwaffen aerial photography specs (the first 10cm versuch for plate films winked at that) and/or the lens formula required too accurate tooling and assembly for a serial production at market prices... Who knows? The Perimetar stated as one of the Zeiss' unfinished masterpieces, and the strong reminiscenses of this lens' concepts clearly evident in the actual, most advanced Leica rangefinder lenses are the best confirmation of it's timeless validity.

 

Another Carl Zeiss Jena wide-angle lens whose evolution required a considerable effort but that didn't reach the production stage was the Sphaerogon, an evocative brand name that refers to his extreme angle of view and massive barrel distorsion: in fact, the Sphaerogon was not an orthoschopic lens but followed the typical equidistant fisheye projection; I guess that the military asked for luftbild superwides for low altitude reconnaisance pictures, and at Zeiss found simpler to compute fisheye inverted-telephoto lenses with huge distorsion that could be set to zero printing the film with a similar lens in reverse position.

The Sphaerogon fisheye lens was another product of Mertè's fertile mind and was defined shortly before the Perimetar; it's optical construction is simple and based on three elements, the first negative, the second positive and the third negative; this latter is configured a glued doublet. Further evolutions from the basic patent embodiment will lead to other three different formulas, more complex and provided with a wider angle of view.

 

The two embodiments of the original patent are really similar in concept; this lens would not have been too expensive to manage, as the front surface is flat and the formula quite simple, with the only exception of the really small rear elements; the aperture was limited to f/8 and the usable field circle theoretically covers 180°; the first sketch shows a easier to polish front lens with flat surface, and was choosen for the first production attempts.

 

The father of all fisheye lenses was the famous "Sky lens" projected by Robin Hill and patented in 1924; this f/16 fisheye lens clutched out a 180° angle of view and was mounted on a special "Hill cloud camera" produced in London by Beck for metereological purposes. The lens is basically a stretched up triplet with the front lens shaped to achieve the desired angle of view, without taking care to the distorsion; all lens makers had to compare with this benchmark, and Zeiss purchased a Sky lens exemplar for it's lens collection (this card is part of the collection's documentation and can testify the acquisition); the Sphaerogon itself is an idea sprouted over this fertile ground.

 

Another proof of  Zeiss underground job engaged on the "Sky" concepts is depicted by a prototype lens Weitiwinkelobjektiv 15cm (5,9") f/5,5 that can be considered an evolution
of this lens and shows the principles applied in the next to come Sphaerogon lenses: the angle of view was tamer (only 110° instead of a whole 180°) and the distorsion didn't reach the typical fisheye figure of 100% but was limited to 23°: a kind of semi-fisheye matching some Pleon models. Another directrix of this prototype was a wider aperture, considering the Sky lens' f/16 a darker figure for aerial pictures (in fact, this versuch is eight time faster that Hill's lens); this evidence strenghten the assumption that the fisheye lenses conceived and produced by Zeiss in the decade between 1935-45 was mainly planned for the military luftbild, with the civilian market set in a secondary row.

 

Crossing the production reports with Mertè's index-cards I was able to recognize twelve different embodiments for the Sphaerogon type (here listed starting from the shortest focal lenght, without following the chronological sequence); as far as we know, the only option planned for a 50 pieces  pre-production trial was the Sphaerogon 1,9cm - 19mm (0,748") f/8: actually survives an exemplar with the serial number 1.503.373, the same that belonged to the Zeiss collection in Jena; this lens bears the 22nd serial of a batch located between 1.503.351 and 1.503.400; Sphaerogons was computed in focal lenghts between 1,6cm - 16mm  (0,623") (for 24x36mm kleinbildformat) and 195mm - 19,5cm (7,677") (for aerial film plates), with apertures f/6,8 - f/7 - f/8 and exploiting four different optical formulas, only the first of whom is described in the available lens patent; the last and most advanced prototype was the V1936 n°18, an 1,6cm - 16mm (0,623") lens with a more complex lens sketch consisting in eight lenses and a more favourable aperture as wide as f/6,8; also this lens was a part of the Zeiss collection shipped to U.S.A. by the Signal Corps.

(to simplify the question, I stated the focal lenghts of all the embodiments in millimeters).

A correct assignment of the exact angles of view is an hard practice: it seems that the f/7 models would be limited to 140°, while embodiments like 1,9cm - 19mm (0,746") f/8 developed upon the first formula (four lenses in three groups), formerly able to spread up to 180°, in trial practice on film delivered a mere 165°; the prototypes developed starting from the third and fourth optical formulas (with an air spaced - three elements front member), on the CADO sheets are claimed for an half-angle of 95° (third computation) and 105° (fourth computation), that means a whole angle of 190° and 210°, but, as we'll see, the theoretical figure was not mantained in the assembled versuch.

 

This draft glances over the various Sphaerogon formulas and the lenses from whom some embodiments' evolution was inspired: as we mentioned previously, the f/16 "Sky" lens by Robin Hill was a forerunner of the 1935 Sphaerogon - first computation; the progress towards the second step (with the front lens replaced by a glued triplet) shares ideas with the first Zeiss Pleon computation by Robert Richter: even if this latter was revealed a couple of years after the six elements f/7 Sphaerogon versuche, probably at this time (1936) Richter was already planning this additional front member to widen his newly computed Topogon, and Mertè has drawn on this computation for the second Sphaerogon step; a further advance provided for an air space in the front triplet, while the first surface became convex; finally, the fourth and last computation (utilized only for the V1936 n° 18 prototype) was fitted with a more complex relay back member, consituted by two glued menisci: the front one is composed by three lenses, in a shape that recalls similar members of the Perimetar embodiments, and a rear one, a doublet similar to the back member of a Sonnar. This lens achieved a more confortable f/6,8 aperture and must be considered the climax of Sphaerogon evolution.

This Sphaerogon 1,9cm - 19mm (0,748") f/8 - the same exemplar found with the Zeiss collection - bearing the 22nd serial number of the 50 pieces' batch reported by Thiele - survived until now;  every Zeiss enthusiast always wondered whether other Sphaerogon embodiments were distributed  outside the factory or not; we can offer a partial answer thanks to the collaboration of the dear friend Dr. Milos Paul Mladek from Wien, who kindly shared the following informations with me.

During wartime, in Austria was published this Ergaenzungswerk book, now exedingly rare, with interesting technical reports upon various lenses, at that time fashionable; some pages were precisely dedicated to a Sphaerogon 1,6cm - 16mm (0,623") f/8, proving that further exemplars was available.

(credits: picture courtesy Dr. Milos Paul Mladek)

 

This picture taken with a Sphaerogon 1,6cm f/8 confirms that the this version, equipped with the first lens computation, couldn't reach in the real world the claimed 180° of view, but achieved a mere 165° or so; as the snapshot was taken with the iris completely open at f/8, we can notice that vignetting is well controlled, maybe as this fisheye lens doesn't follow the Lambert Cos⁴ law.

(credits: picture courtesy Dr. Milos Paul Mladek)

 

 

The sample picture was followed by this unprecedented index-card with the section drawing of the lens (clearly showing the first optical computation) and other aberrations' graphs that curiously states a 90° half angle of view.

(credits: picture courtesy Dr. Milos Paul Mladek)

These data are shared with the previously described Sphaerogon 1,9cm f/8, equipped with the same optical outfit.

Another interesting prototype catalogued by the Signal Corps in 1947 was the V1936 n°18, the most complex, fast and interesting version among Sphaerogon versuche; this lens - together with the other items of the Zeiss collection - was soon after entrusted to the Burke and James society and kept in store for a couples of decades; after that the lenses was discharged by the military,  put on the market like useless scraps, and mostly purchased by the Burleigh Brooks House. The following destiny of the V1936 n° 18 1,6cm - 16mm (0,623") f/6,8 was misty, but thanks again to the contribution of  Dr. Milos Paul Mladek we can show unpublished pictures of this lens.

 

The Sphaerogon series was conceived during the epic years that marked the transition between Zeiss Ikon Contax I and Contax II cameras; the V1936 n° 18, produced in spring 1937, would have been the forerunner of a 1,6cm - 16mm (0,630") f/6,8 fisheye lens for the Contax II outfit, allowing a full coverage of the 24x36mm format with an angle of view from corner to corner of 180°.

(credits: Bessa camera and lens courtesy Leicashop - Wien)

 

The V1936 n° 18 prototype lens was the only one to be fitted with the most advanced Sphaerogon computation, with an air-spaced front member plus a convex front surface (like in the third version) and a complex rear relay member that's a complete lens by itself; this shape was choosen to rise up the aperture from f/8 to f/6,8.

 

Interestingly, when photograped by Signal Corps, in 1947, this lens clearly lacked a specific mount and a shutter; after that, during it's enigmatical life, the V1936 n° 18 was equipped with a Deckel leaf shutter and fitted on a Voigtlaender Bessa rangefinder camera whose lens and focus assembly was stripped out, carving the metal body to achieve room enough for placing the leaf shutter as close to the film plane as needed by the lens specs. With this new configuration, the lens is placed on a fixed focus setting (exploiting it's huge depth of field) and exposes a 120 film, allowing the full utilization of the complete image circle.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

The lens in the actual shape with it's original cap, never lost over  half a century! You can notice that the knurling on the lens cap match the ones on the aluminum ring of the lens.

(credits: picture courtesy Dr. Milos Paul Mladek -  item courtesy Leicashop - Wien)

 

The lens still appears in excellent state of preservation, almost like new.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

The raw versuch mount is fitted with two aluminum knurled rings: the lower  works as a catcher to hold the lens and the upper is the iris control ring, with references engraved in black.

(credits: picture courtesy Dr. Milos Paul mladek - item courtesy Leicashop - Wien)

 

The front rim of the lens has a diameter of more or less 3"; the lens' data are clearly engraved on a black screwed ring surrounding the fron lens element, where we can read: Carl Zeiss Jena Nr. 2030705   V1936 Nr18   1:6,8  f = 1,6 cm

(credits. picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

 

As anyone can assume looking at this close-up, the lens elements appears to be coated, an impression borne out by Dr. Milos Paul Mladek, who had this lens in hand for some time; also, apart from some iridescences on the lens' outline (maybe caused by chemical depositions due to some volatile substance evaporated from glue or other materials and kept over the lens by the cap fitted for years), the purple color of the reflected light (instead of the typical cyan shining) suggests a multi-layer coating, with at least two layers; considering that the lens was produced in early 1937, when the newly discovered Zeiss vacuum-coating system was settled as a patent for military use only, we can argue that this lens was intended not only for the civilian market but also as a prototype for further embodiments assigned to Luftwaffen aerial cameras, eventually with boosted focal lenght and a bigger film format; obviously these latter are only conjectures.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

The iris control ring manages aperture settings between f/6,8 and f/32: the closest value is unusual for an extreme wide angle of it's era, and let me guess that - maybe - a service on leaf shutters (lacking the fastest shutter speeds usual on curtain focal-plane devices) was planned from the beginning.

(credits: pictures courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

This close-up discloses two interesting details: the iris  with a mere five blades (maybe to simplify the constuction of this small device) and the light baffles engraved on the inner ring that connects the wide front member to the relay lens in the back, a further detail that underlines the meticolous attention to light reflections control.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

 

This picture of the "Bessa mother" with the film lid open discloses the signs of the somewhat coarse "Frankenstein-style" job; the arrow withnesses that the backfocus clearance was reasonable, surely larger that expected from a brief look to the section drawing of the lens.  The crude adaption leads to massive light leaks when winding the film.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Leicashop - Wien)

As we said, the V1936 n° 18 adapted on a medium format Voigtlaender Bessa body allows the lucky owner to exploit the whole image circle projected by the lens, without cuts, as explained by the graphics; in 2004, during the few hours (SIC) this lens stopped in Vienna, there was time for a rushed photographic test on a Kodak T-MAX 400CN - 120 roll-film. The following trial image is an exceptional record, as it' actually the only available shot taken with this epoch-making lens since 1937.

The test shot taken from a balcony with the V1936 n° 18 on medium format Bessa and 120 film reveals massive light leakages on all the frame but the area inscribed inside the lens' mount ring: this particular, pressing against the film, seals this sector from unwanted  light beams. The other marks that partially affect the frame were exposed during the film advance.

(credits: picture courtesy Dr. Milos Paul Mladek - remastered)

 

The positive image, apart from unwanted light leakages, clearly prefigures a fisheye shot; the effective angle of view is a true 180°, as the flower-holders' balcony line was a few centimeters from the lens, on the same vertical plane with the camera, and these subjects are entirely reproduced, from corner to corner. You can notice the great flare resistance against straight sunlight, a direct corollary of this lens' advanced coating.

(credits: picture courtesy Dr. Milos Paul Mladek - remastered)

 

The graphics show the original 24x36mm film format, inscribed inside the whole circle recorded on the wider film strip: after having gauged the diameter of the circle, I can confirm that the lens exactly frames a 24x36 image, no more. Inside the square mask you can recognize the image actually produced by a Contax, if we could mount the lens on this camera.

(credits: picture courtesy Dr. Milos Paul Mladek - remastered)

 

This index card reports the section drawings and the construction data of the first, second and fourth Sphaerogon computations; unfortunately I was not able do get the same informations about the third specification, similar to the second but with an air space between the first and the second lens and a convex front surface.

 

This further sheet summarizes the construction and functional data of the only two Sphaerogon lenses survived till now, the 1,9cm - 19mm (0,748") f/8 (serial n°1.503.373) and the V1936 n° 18 1,6cm - 16mm (0,630") f/6,8 (serial n° 2.030.705); the effective angle of view reported by the graphics is 180° (90° + 90°) for the first and an astonishing 210° (105° + 105°) for the second lens, but - as previously shown - the effective coverage must be considered in the order of 165° and 180°, respectively.

UPGRADING 24/05/2010



The V1936 n° 18 brief data shown above (and reported by Edward K. Kaprelian in his 
comprehensive article written in June 1947 for the Journal of the Optical Society of America)
was actually recomputed by the friend Pierre Toscani, and regrettably they must include a
wrong parameter (maybe an unwanted typing mistake?), as - drawing the section - the sketched shape doesn't match exactly the profile of Mertè and Keprelian's original design (even if the deviation is not massive); also, the raw data supposed for a 100mm focal lenght, if computed, lead to an effective 86,79mm at full aperture and 86,88mm when respecting the Gauss conditions; finally, using the kaprelian's raw data, and assuming a true focal lenght of 1,6cm (as engraved on the barrel), for a true 180° angle of view we need a 45,9mm image circle, that's fairly larger than the former one intended for the 24x36mm format (43,27mm); keeping this latter, the angle of view should be a mere 160°. Summarizing, at now it's impossible to match the exact, original shape of the V1936 n° 18 but we are only able to get a close copy, but not perfect, of this lens' formula.

The drawing in black is the result of Pierre Toscani's computation of raw data, while the red one underlines the original sketch by kaprelian: as you can see, some radii and spaces look clearly unmatching.

(drawing by Pierre Toscani)

The results of the computation for an 1,6cm focal lenght lead to a lens whose lenght between the focal plane and the front lens vertex is 64,74mm, with a 19,87mm backfocus clearance
and a 70,60mm usable front lens diameter; H and H' points are located at both sides of the
rear member with a 14,02mm space between them. As you can see, tracing the rays to
match the corners of the 24x36mm film format, the Theta half-angle is a mere 80°, while
assuming Theta = 90° the rays hit the focal plane slightly outside the limits of this field; as
the actual pictures taken with the lens show a true 180° of view inside the 24x36mm format,
it's possible that this problem could depend from the aforementioned typing errors in the raw data reported by Edward K. Kaprelian.

(Many thanks to my friend Pierre Toscani for this stunning graphic job)

 


END  OF  THE  UPGRADING  24/05/2010

In the CADO sheets' collection I found the profiles of six among Sphaerogon exemplars; sadly most of them are almost unintelligible, and I must share my indignation with  Dayton guys that kept stored for sixty years such a precious piece of history without worrying too much about it's preservation.

 

The V1935 n° 12 was a prototype based on the first Sphaerogon computation (the first embodiment of the original patent); it's assigned serial number would have been 1.699.641 and the focal lenght of 3,8cm - 38mm (1,496") could display a 165° image on a medium format film.

 

This second card relates to a lens identical to the previous, but not indicated as a versuch: maybe it could be the pre-production configuration; we can notice that the rechnung data differs only within eleven days from the V1935 n° 12 references (09/05/1935 and 20/05/1935). Interestingly, the aberration curves appears slightly different.

 

These data belongs to the widely debated V1936 n° 18; the drawed curves extend up to a 105° half-angle of view, a figure not reached in the image test.

 

The fourt lens in the sequence is the Sphaerogon 36,2mm (1,425") f/8; this model shares the third optical formula (six lenses, front member with air spaced elements and convex front surface) and it's focal lenght is rated in millimeters; this lens directly bears the "Sphaerogon" brand name, so maybe it was planned for production; the graphs show an available half-angle of 95°, that means a whole 190°, but on film the effective angle exploited would not exceed 180°.

Another sketch recovered in the CADO database clearly show the section of the Sphaerogon 36,2mm (1,425") f/8.

The fifth sheet hatches the outlines of a Sphaerogon  36mm (1,417") f/8, very similar to the previous one; as you can see, several Sphaerogon projects orbited around focal lenghts in the 36-38mm (1,417" - 1,496") range, maybe because these figures matched the covering of a particular and desired film format (maybe a square luftbild plate?).

The sixth and last card is dedicated to a Sphaerogon 4,1cm - 41mm (1,614") f/7, a particular lens with the second optical formula embedded and a maximum aperture of f/7; uncording to further notes gleaned in Richter's memos, this  kind of lens should limit it's angle of view to a tamer 140°; even this exemplar is brand named Sphaerogon. We have reports of a lens with the same optical sketch and 12cm - 120mm focal lenght  (with serial number 1.756.712), assigned to larger film sizes. We have no records about the big one, the Sphaerogon 19,5cm - 195mm (7,677") f/8, but it's focal lenght - in the typical 20cm range - suggest a final destination as lens for aerial photography on a Reihenmesskamera, together with "brothers" like the Topogon 20cm f/6,3 or the Pleon 20cm f/6,3.

Two Sphaerogon prototypes were reviewed by the mourned Edward K. Kaprelian in the ambit of his article "Recent and Unusual German Lens Designs", published in June 1947 on the "Journal of the Optical Society of America", emphasizing the 210°  theorical angle of view of the V1936 n° 18. This lens: what an history's crossroad, isn't it?

 

As nothing dies completely, we can still find a Sphaerogon heritage in the Asahi Pentax Takumar fisheye 18mm f/11...

Another example of an extreme wideangle computed by Zeiss technicians during late '30s was the famous Pleon; consistent reports whisper about a massive utilization of this lens by Luftwaffen for aerial reconaissance during the last years of war, but - one more time - the effective and documented production flats almost near zero!

 

Recognizing the data about the Pleon, in the original project were included two embodiments: a fisheye 19,2mm (0,756") f/6,3 (180° on 6x6cm format) and a semi-fisheye 80,4mm (3,165") f/6,3 (136° on aerial picture formats, with a barrel distorsion limited to 49%); there was an 80mm (3,149") f/6,3 versuch strictly derived from the second embodiment but never produced; finally, two versions was planned for production: a 72,5mm (2,854") f/8 (semi-fisheye, with an angle of view of 152° and a barrel distorsion rated at 65,5%) and a 200mm f/6,3 (semi-fisheye for a bigger film format, with 136° and 49% barrel distorsion).

For the Pleon 72,5mm (2,854") f/8 was expected a 50 pieces production (between the serial numbers 2.801.301 and 2.081.350), but we have no evidence that the assembly lines effectively ran; about the gigantic Pleon 200mm f/6,3, only three exemplars was reportedly produced, and - about that - I can confirm the sighting of one exemplar in Wien in mid '90s.

 

The Pleon was computed by Robert Richter in 1938 as an evolution of it's previous masterpiece, the Topogon; Richter added a front member to the Topogon's nutshell and achieved a fisheye or semi-fisheye lens. The first embodiment of the patent shows a front glued triplet (with two flat surfaces) that's similar to some Sphaerogons' front members; it was optimized for a 19,2mm (0,756") focal lenght and can be labeled as a true fisheye; the front member includes two outer elements in dense Krown SK10 glass and a central element grinded from dense Flint SF2 glass. The second embodiment,  an 80,4mm (3,165"), is equipped with a simpler front module, with two glued lenses and a flat first surface. The most difficult task producing the Pleon relates to the reduced dimensions of the inner menisci in the rear "Topogon" module, whose huge curvature and thinnes (and the use of difficult to manage SF1 dense Flint glass) were truly time wasting and required particular tooling attentions.

 

The shorter among Pleons reaching the production stage was a 7,25cm - 72,5mm (2,854") f/8 lens that equipped the Zeiss Reihenbildkamera 7/1818; as you can notice, Richter neglected the 180° formula, whose standard fisheye projection was maybe too difficult to manage in an orthoscopic image while printing, and concentrated to the semi-fisheye option; this lens allowed a 152° angle of view with 65,5% barrel distorsion; the lens formula make use of three different kinds of glass with a budget-saving front lens produced with a common borosilicate Krown and equipped with an easy to grind flat surface. As said, the report talking about a 50 pieces production can't be endorsed.

 

The "big Bertha" among production Pleons was the impressive 20cm - 200mm (7,874") f/6,3, an heavy and bulky lens whose front lens skimmed a diameter of 40cm (15,75"). A dear friend of mine, around 1995, had in hand one of the reportedly three Pleon 20cm f/6,3 ever produced (what a lucky opportunity!), and his brief description confirms the destination to military purposes only: in fact: 1) the lens barrel was painted in military olive  2) the lens was coated, with the red "T" engraving  3) there was no "Carl Zeiss Jena" engravings but only a wartime inscription (maybe "Bauart plc", the friend doesn't remember exactly); he reports a late wartime serial number in the 2.7xx.xxx range (matching the official figure of three exemplars between 2.757.898 and 2.757.900). Further evidence of the exclusive military purpose is provided by the glasses' specs: in last years I minutely investigated the wartime Zeiss lenses for infrared devices (UR-objektive, for the UHU project), and I descovered a common denominator: almost all these lenses (not achromatized for white light but only for a narrow wavelenght in the 1000nm range) featured only TWO kind of glasses: a low refractive / low dispersive borosilicate Krown and an high refractive / high dispersive dense Flint; this essential selection allowed to easily manage the storehouse during the unpredictable war events; in  the Pleon 20cm f/6,3 the front member was air spaced and the SF2 glass of the 72,5mm version was changed to SF1; in this manner both the front member and the rear "Topogon" module adopted only a kind of borosilicate Krown and a kind of dense Flint as raw materials; a complete achromatization for white light was not possibile, but this Pleon featured a deep yellow low-pass plate behind the lens to enhance the contrast in aerial photography, and a correction for white light was useless. This embodiment covered a field of 136° with a 49% barre distorsion; special enlargers was accomplished to print these Pleon aerial plates and where equipped with a reversed version of the same lens to annul the strong distorsion. This lens was fitted on a Zeiss Reihenmesskamera and was renowned among the Allied Forces (we must remember that this lens' patent was filed in 1939 at the United States Patent Office, and everyone could read it's claims and data), to such an extent that Carl Nelson, U.S. representative of the Allied Committee on optical reparations, considered a priority to acquire the control of Pleon's production shortly after the German's surrender to recycle this device in the last was effort against the Japanese (operation Downfall).

 

This index-card relates to the Pleon 7,25cm - 72,5mm (2,854") f/8 and shows a far better aberrations correction (apart from distorsion, obviously) on the Sphaerogon figures; maybe this is the reason for what the Pleon was preferred for aerial porpose and the military Sphaerogons did never become a reality. The rechnung date (14/02/1940) is still legible.

The second index-card proposes the available information about aPleon prototype 8cm - 80mm (3,150") f/6,3 never listed before; it's rechnung date is 17/07/1939 and the optical formula strictly derived from the second embodiment of the Richter's patent; the maximum aperture forced to f/6,3 somewhat penalizes the aberration controls, and the f/8 version seems to be more corrected.

Interestingly, most of optics experienced went on for years considering the Pleon as a true 180° fisheye, while only a never produced patent's embodiment matches these specifications; the following paragraph was extrapolated from a military document written by Robert S. Capriani of the Australian Land, Space and Optoelectronics Division - Electronics and Surveillance Research Laboratory , entitled "Missile detection by observation of differential motion in an image sequence"; this document was published in 1995 and - as you can read - still claims the Pleon as a 180° fisheye.

 

If we can learn a lesson from this fascinating saga of CZJ prewar superwides, must say that such a great amalgam of passion, effort, skill, talent and boldness heroically crashed at full speed against the annihilation wall: great inspirations, genial minds, complex draws and oneiric visions... all for what? Some versuche, an handfull of produced lenses, no more, all erased by the unpredictable events, like in the famous, sad Hauer's monologue from the "Blade Runner" cult movie; paraphrasing the human nature, as these lenses are famous and acclaimed in death, so they floated as unvisible ghosts during lifetime, barely reaching the existence ground; this tragic fate is a leit motiv of all Zeiss lens production: jewels carpeted with endless shine but often mortified by insignificant production figures. We also can't get out of recording that, after the experience with Sphaerogon and Pleon lenses, Zeiss stopped producing fisheye lenses, an abstinence broken only in 1972 by the Zeiss Oberkochen F-Distagon 16mm f/2,8 T*.

A Zeiss F-Distagon 16mm f/2,8 T* in Contarex mount: again a Zeiss fisheye lens so far from the '30s ancetres!

An appendix must be reserved to other rare and interesting german fisheye lenses of the '30s; particulary, in 1929 (this year is claimed as correct but can't match the CCX engravings)  the firma Hugo Meyer Goerlitz realized an almost unknow fisheye lenses, reportedly calculated by Dr. Paul Rudolph; here you are some picture of this rare item.

The mysterious fisheye by Meyer Goerlitz was realized on demand for the military (as stated by the "CCX" engravings in the rear mount, a military code meaning "Hugo Meyer & Co."); at a brief look the optics appear complex. The lens comes in LTM mount, fixed focus and without iris.

 

Another milestone is the famous AEG Weitwinkelobjektiv 1,7cm - 17mm (0,669") f/6,3, a fisheye lens conceived by the AEG - Berlin during 1935 for metereological purposes (clouds recording); this lens was claimed for an angle of view exceding 180° (the CADO memos suggest  200°) and it's optical formula is a true benchmark, the mother of all modern fisheye lenses.

The AEG Weitiwinkelobjektiv was effectively a spartan camera in perfect working order, with a Deckel leaf shutter, a confortable tripod mount, a barrel and a machined metal plate fitted with hookings for a Zeiss ikon ground glass, replaceable with film plate holders.

(credits: picture courtesy Dr. Milos Paul Mladek - item courtesy Westlicht Photographica Auction - Wien)

 

The lens shape resembles modern fisheyes but the elements were uncoated; the base plate is roughly finished and bears the "AEG Weitwinkelobjektiv" engravings.

(credits: picture courtesy Dr. Milos Paul Mladek - remastered - item courtesy Westlicht Photographica Auction - Wien)

 

Another view showing the Zeiss Ikon ground glass with spring loaded hood.

(credits: picture courtesy Dr. Milos Paul Mladek - remastered - item courtesy Westlicht Photographica Auction - Wien)

 

The rear lens barrel is unespectedly milled to polished brass; the lens depitched a circular image with equidistant projection and the close view oh the rear plate confirms the impression of roughness and tame trimming; the rear plate is machined from a solid block.

(credits: picture courtesy Dr. Milos Paul Mladek -  item courtesy Westlicht Photographica Auction - Wien)

 

The optical sketch of the AEG fisheye represents the embryo of modern fisheye lenses: it's formula was immediately shared with the Nippon Kogaku's technicians and in their first fisheye camera, released in 1938, the fisheye-Nikkor 16,3mm f/8  was a true clone of the AEG lens, and the first fisheye-Nikkors for the Nikon F outfit (like the 8mm f/8 and 7,5mm f/5,6) shared obvious analogies with the original AEG and Nikkor fisheye lenses.

 

The original index-card of the AEG lens n° 620 538, the 1,7cm - 17mm (0,669") f/6,3, states that we are talking about a " weitwinkel ueber 180° "; the five lenses are grinded using two borosilicate Krown glasses (identical, for the first and second convex lenses), a dense Krown, a Flint and a dense Krown again.

These pictures were taken by Sten Vondrak with the AEG Weitwinkelobjektiv 1,7cm - 17mm (0,669") f/6,3; the second, zenital image clearly shows that the effective covering power can reach the claimed 180° image field; the contrast is on the low side (the optics are not coated) and the lens cast a 50mm image circle suitable for 120 type film.

 

These enlarged image crops (the left one from axis and the right one from the corners) show that the resolving power was decent, if not exceptional, in the center of the image, but quickly trailing on field so the far corners were strongly aberrated; anyway, as the lens was intended for clouds documentation, an even resolution was not the primary task.

Thanksgiving

Many thanks to the dear friend Dr. Milos Paul Mladek from Wien, a worldwide famous expert, auctions consultant and collector, for the precious pictures he shared with me, the priceless informations and his disinterested friendship; many thanks also to Leicashop - Westlicht Photographica Auction in Wien for the availability of extremetely rare items; the same for the french friend Pierre Toscani, for his skillfull computations and mervellous graphic designs; warm hugs to my dear friend Dr. Klaus Schmitt from Weinheim, the exedingly famous expert and collector of special purpose lenses, for providing pictures of the Meyer fisheye lens and for the rediscovery of the Zeiss files; many thanks also to Sten Vondrak for sharing the pictures taken with his AEG fisheye lens; a grateful thought also for another dear friend, the Dr. Pierpaolo Ghisetti from Modena, a great conoisseur of the Zeiss world and collector, who let me take pictures and also play a bit with his rare F-Distagon Contarex 16mm f/2,8; last but not least, a touched, respectful and thankful thought for Robert Richter and Edward K. Kaprelian, which collected and handed down for the next generations a plentiful material of invaluable importance about these fashinating prewar Zeiss lenses.

(Marco Cavina)

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