Wednesday, June 5, 2019

The Ambi Silette: Agfa’s most ambitious 35mm rangefinder camera

Agfa Ambi Silette with 50mm f2.8 Solinar and original lens hood

The history of West Germany’s camera industry in the 1950s is one tightly bound to the leaf shutters manufactured by Deckel and Gautier. It is a story of initial success leading to technical dead ends followed by the ascendency of the Japanese camera industry. The Agfa company was an intimate part of this classic business case study that explains why companies (and industries) can ultimately fail due to their original success. The Ambi Silette, Agfa’s only interchangeable lens 35mm rangefinder camera, was part of that story.
    While labor costs, too many small companies and changing buyer taste were factors, the inherent optical limitations imposed by a leaf shutter were a major factor in the ultimate failure of West Germany's camera industry.
The why has to do with optical design. The glass elements in a lens shape light entering the front of a lens and bring it to a single point: the “optical center” of that lens. From there the light spreads, emerging out of a lens’ rear element to form a sharp image a focal length away. Because the cone of gathered light is smallest at the prime nodal point, even a leaf shutter with a small opening set at that point can pass that light through without causing vignetting.
Top of Type II showing modified advance and three-lens frame selector.
    But placing a leaf shutter behind the lens in order to gain full lens interchangeability  requires either a much larger shutter opening and/or special formula lenses that place their rear element close to the shutter.
    Zeiss Ikon’s Hubert Nerwin first ran into this problem while designing the earliest interchangeable lens, leaf-shutter 35mm camera, the Tenax II, in the 1930s. He was able to avoid vignetting by limiting the format to 24X24 and using deep-fitting optics that set their back elements right in front of a larger opening, medium-format, Compur Rapid shutter. Despite this approach, the Tenax offered only one short telephoto lens with a modest maximum aperture of f4.
    By 1950, public demand was back world-wide for German-made cameras. Dozens of companies in West Germany were competing for a slice of a growing market for high-quality 35mm cameras that were a cut above the American-made Argus C-3. Yet, except for the top, professional level Leicas and Contaxes, and one single lens reflex, the Wirgin Edixa, none of these cameras came with focal plane shutters. Everyone else came with either a Deckel-made Compur or a Gautier-made Prontor or lower quality variant. The reasons were several. Zeiss Ikon, the giant firm that controlled much of what West German camera companies did or did not do, owned both Deckel and Gautier, and it was determined that both those shutter companies would prosper.
    With limited resources for development and experimenting, it was easier for the many smaller West German camera companies to buy and install leaf shutters with consistent and predictable performance rather than develop a new shutter design. As long as their cameras offered only fixed, normal lenses this was not a problem. But the moment interchangeability of lenses became essential, optical design, vignetting and mounting became a problem.
    Agfa was always a film and chemical company first. It got into the camera business as a way to promote its films. This followed the same logic that Eastman Kodak was to use so successfully. Customers who bought cameras with the same brand as a its film, tended to buy that company’s film. Agfa’s earlier 35mm Karats and Solinettes were followed by the Silette series in 1953. The Silette cameras started as simple, all-metal, non-interchangeable lens 35mm cameras. Cameras with bellows or collapsible lenses were losing popularity as photographers looked for more features, easier to use controls and more solid construction. The Silettes offered such modern conveniences as a hinged back, a fixed takeup spool, single-stroke, film advance and a modest range of shutter speeds. The series sold well and Agfa soon added more advanced models with unit-focusing lenses as fast as their 50mm f2 Solagon, rangefinder focusing and built in meters. Agfa soon bragged that it had sold over a million Silettes.
    In 1956, Deckel made available a behind-the-lens version of its new Compur Synchro shutter with full MX flash sync and a built-in self timer. At Photokina that October, several West German camera companies announced new models to take advantage of this shutter. Agfa, with its Ambi Silette, was one of them.
Advertisement for Ambi Silette from American Photo magazine from 1958

    Agfa started marketing the Ambi Silette in Europe in January 1957. It appeared in American advertisements starting in May 1958 and continued to be sold until 1961. By that point, Agfa was putting its high end efforts into leaf-shutter single lens reflexes—none which proved successful.
    The Ambi was the first post-war camera that Agfa would sell in the United States under its own brand. All its previous 35mm models had been rebranded and sold by the Ansco Corporation as Memars or Karomats. A study of serial numbers indicates sales of over 80,000. Despite this moderate success, little has been written about the Ambi, and detailed histories as to who designed it and what its manufacturing involved do not seem to exist.
 I am a camera historian with some expertise concerning early Nikons and Zeiss Ikon’s Tenax II camera. In 2018, I was ready for a new camera to investigate and turned to the Ambi due to its sparse history and unique characteristics. Several its features appealed to me: its full-featured finder, interchangeability of lenses and its unique lens mount. Again, as with the Tenax II, this was a camera with a rather complete but limited system of accessories. I just might be able to buy everything Agfa ever made for the camera and at moderate cost.
    In October I came across an Ambi complete with all four of its lenses, including the rare 130mm f4 Color-Telinear. I bought the set, then spend several weeks struggling to get the shutter to operate correctly. Once fixed, the camera proved a pleasure to use, and I continue to enjoy taking pictures with it.

Open back of a first model Silette and Ambi Silette showing design similarities. Note the large eyepiece on the Ambi.
Oveehead view of Type I (top) and Type II (below)
Type II viewfinder with larger, wider rangefinder
Type I viewfinder with single range/bright-frames window
The Camera:
The Ambi Silette is clearly part of the Silette series. In size, shape and its numerous features, it is like all other Silettes up until style changes finally came in the late 1950s. The interiors of the Silettes are the same, the awful rewind knob with its nearly impossible to change film speed indicator is the same. The single stroke film advance and the manual reset frame counter are the same. The finder hump in the middle of the camera top is the same. Even the finish and leather-like body covering are the same. It is what is under the finder hump of the Ambi and what’s in front that are different and unique to the Silette series.
    A large eyepiece offers considerable eye relief and most of the bright frames can be seen even with glasses. The .73X view is clear and the rangefinder spot is easy to use, if not quite as sharp as a Leica’s. A sliding switch on top changes the auto-parallax correcting bright frames to cover 35mm, 50mm and 90mm lenses. This is one of the best rangefinder/viewfinder combos ever put in a 35mm camera, a high-end feature comparable to the finders found in M-series Leicas and late-model RF Nikons and Canons. And at $125 American, it was a lot less expensive. Agfa must have been proud of the Ambi finder for it protected its front glass with a hinged, velvet-lined cover that you must flip up out of the way in order to see anything. Annoying for many, but a feature I have gotten used to.
    The second unique feature is the Ambi’s clamp mount. Place one of the Ambi lenses in the opening, press and turn clockwise until you feel the lens seat, turn a few more degrees until the lens clicks, and you are set to go. Yes, you can line up a red dot with a red dot, but that is optional. What is happening? A notch on each Ambi lens engages a prong in the camera mount. Turning on the lens causes two circular blades to spring out and lock into a groove on each lens. The nearest equivalent would be the clamp mount found on Arriflex movie cameras. The mount is fast, secure and easy to use. Unfortunately, nobody else ever used it, so the four Agfa Ambi-mount lenses are all a user has to draw on [But see my earlier entry on the Schneider Curtagon in Ambi mount.]
The four Agfa lenses for the Ambi Silette
  When the Ambi came out, Agfa offered only three lenses: a 35mm f4 Color Ambion with stops to f22 and focusing down to three feet, a 50mm f2.8 Color Solinar with stops to f22 and focusing down to three feet and a 90mm f4 Telinear with stops to f32 and focusing down to six feet. No need for accessory finders. The finder covered all three of these.
Certainly a modest range of optics with small maximum apertures and no close focusing capability. It is unfortunate considering the professional level of the finder, but Agfa would never offer faster lenses.
    These lenses featured both the good and the bad. The good part is that all are nicely finished and solid with aluminum barrels over brass helicoids and chromed rangefinder feeler plates. All three take the same 35.5mm screw-in, 37mm press-on filters. All have a stationary milled ring right behind the aperture ring allowing easy gripping for mounting. The aperture clickstops with half-stop settings are precise and firm. Distances are engraved in both meters and feet. I can find no fault with the lens construction. The lenses shipped in plastic-and-metal “tropical” cases that seal the optics from dust and humidity and hold silica grains in the bottom to absorb moisture.
    Handling is another issue. The Color Ambion is a Tessar formula lens. It is small, and light. Did I say small? The focus ring is less than two millimeters wide with sharp milling. Raising the viewfinder visor blocks any view of the tiny distance scale. To add to the challenge, you must rotate the focus ring over 340 degrees to go from infinity to the closest point. This makes follow-focusing impossible.
    The 50mm Color Solinar is Agfa’s version of a Tessar. Good lens, nice images, better handling than the Ambion, but a 270 degree rotation slows a user down. On the other hand, the five-element 90mm Telinear has a large, easy-to-grip focusing ring, easy-to-read numbers and a short 60 degree throw that make it a pleasure to focus.

The 130mm f4 Color Telinear with modern shade and dedicated finder

  Agfa added the 130mm f4 Color Telinear to the Ambi’s lens range in 1959. Same quality construction but with a huge front element (taking 62mm filters) and focus down only to nine feet.
    All four of these lenses are capable of excellent results with good, modern saturation of colors and good correction. But why did Agfa restrict its choices to these modest aperture optics within a limited range of focal lengths? The answer may lie in the limitations of the behind-the-lens Compur shutter. With an opening of only 25mm, vignetting would be a problem with any high speed lens. Close focusing that would move a rear element too far from the shutter opening could only compound the problem. Or, perhaps, aiming its marketing at advanced amateurs, particularly women, Agfa felt these optics were enough. Other companies, such as Voigtländer and Kodak’s Retina series, ran into similar challenges but did offer faster lenses, but perhaps had more lens-designing resources
    There are three versions of the Ambi Silette: a Type I, a less-common Type II and a rare special deluxe version that came covered in snake skin but otherwise was the same as the Type I. The first version was replaced by the second in 1959 without fanfare about the same time that the 130mm lens came out and the change may be related to that lens.
    Fitting the rangefinder/viewfinder into the same width as other Silettes limited the space available for a rangefinder. In the first version, the rangefinder was centered in the bright-frames window yielding only a 35mm base. When combined with the .73X magnification of the viewfinder, an effective rangefinder base (ERB) of only 25.5mm was the result. Barely enough for focusing accurately with even a slow 50mm f2.8, but not enough for a 130mm lens, even with its close focus limited to three meters. In the Type II, the rangefinder window was moved to the right edge of bright frames window, increasing the base to 40mm and increasing the ERB to 29.3mm. A small gain, but an improvement.
    The second version featured a number of other minor changes. Two features that definitely improve handling were one) the addition of strap lugs. They are narrow but appear to be steel, so should do the job. Two)was changing the shape of the advance lever so that it projects off the back of the body. Much nicer.
    Other minor changes: The frame counter changed from an additive to a subtracting counter. The film type reminder changed from black on white to an easier to read white on black. The frame switch changed from a 35-50-90 progression to a 50-35-90 progression. In the Type I, the 35 frame appears at all sittings. In the Type II, the 35 frame disappears at the 90 setting.

The Ambi Silette is typical of many leaf shutter cameras from the 1950s, and buying one always involves taking a chance. Sometimes everything works; sometimes a shutter won’t work, sometimes a lens won’t focus, sometimes the film advance will be messed up, the frame counter will be stuck and sometimes the rangefinder will be out of caliper. It is difficult to predict what will wrong. Of the four lenses I got with my first Ambi, the Ambion focused but felt dry, The normal’s focus was smooth as factory-new. Both the 90 and the 130 were stiff and took a lot of exercising and application of lighter fluid to get them turning. I own three Ambi Silettes now—a Type I with poor cosmetics but shutter and lens work pretty well. A second Type II has a good shutter but the lens focus feels dry. Rangefinder was off, so I had to reset that. As I noted above, the first Type II I bought has a non-working shutter. I had to take off the shutter, open it up, clean and exercise it until finally it decided to behave. Fortunately there are several on-line resources that can take you through the whole process step-by-step with numerous pictures for guidance. It is hard to “break” a Compur shutter, but a lot of them require cleaning in order to get their speeds back within tolerance.
    So how do I like the Ambi Silette? Good camera with excellent handling. Easy to take pictures with and capable of excellent results. The shutter speed ring is hard to read and projections cut into my fingers when I change the settings, The rewind knob is a nuisance. The advance lever has too much travel and no racketing capability. The 35 and 50mm lenses require too much turning to focus. But this is minor stuff. Overall the Ambi is a fun camera. It is solid without pulling at my neck. The finder is a real pleasure. The shutter is quiet and vibration free. The little slide-out shelf underneath to keep the camera level on a table surface is cute and practical. The Ambi proximeters, singly and combined, can take either the 50 or the 90 from three feet all the way down to 18 inches.
Proximeter on the Ambi Silette
    For a top-of-the-line camera in its day, Ambi Silette often go for little money on ebay today. The fixed lens Karat 36s and Solinettes generally seem to demand higher prices. The challenge is to find an Ambi that is working. Then, if a film user wants its interchangeable lenses and does not need high speed optics, this might be exactly what he or she needs.

 Some pictures taken with Ambi Silette.

Friday, May 10, 2019

Schneider lens for Agfa's Ambi Silette camera

A Color Ambion 35mm lens and the 35mm Curtagon for the Ambi Silette
The Ambi Silette 35mm camera was Agfa’s most ambitious rangefinder camera featuring fully-interchangeable lenses, a large, beautiful rangefinder-viewfinder with auto-compensating bright frames and a Synchro-Compur shutter with speeds from one second to 1/500th. However, Agfa never offered more than four focal lengths and the fastest was the 50mm f2.8 Color Solinear. The only wide angle was a modest 35mm f4 Color Ambion barely focusing down to a meter.
    So where did this Schneider 35mm f2.8 Curtagon come from? It certainly did not start life as a Ambi Silette lens, but it mounts, focuses and changes f-stops flawlessly. Alas, no rangefinder coupling, but being a wide angle should allow a calculated guess to provide correct focus most of the time.    The front end back to the aperture control is finished in a brushed chrome. In appearance it matches that of the Curtagons made for the Retina Reflex from the time period that the Ambi Silette was on the market. The word Compur engraved on the focusing ring would tend to confirm this.
    The Aperture ring and the Ambi mount are a brighter chrome and made from a heavier stock, either chromed brass or stainless steel.    At first one might guess that this is someone’s custom job run off on a lathe. But if so, why does the rim of the aperture ring sport a marking “Agfa – Typ 44017/100?”
    Underneath the focusing ring is an engraved image of a flashbulb that acts as marker for a range of f-stops. Is this a flash guide? If so, for what type of bulb?
    No filter threads, but 40.5mm slip-on accessories fit. It focuses down to two feet. It does not appear to vignette. So, here is a close-focusing, faster lens that uses a more modern five or six element retrofocus design. So why did Agfa stick to the slow, Tessar-based Ambion?
    Who knows? But I intend to enjoy this lens and find out what it is capable of producing.

Looks like a Retina Reflex lens
The Agfa marking on aperture control ring.
Ambi Silette clamp mount.
Flash guide marking on bottom of focus ring.

Lens' beauty ring.

Wednesday, September 5, 2018

My Super Ikonta A

 The Super Ikonta A, Zeiss Ikonta's smallest 120 film folder.


The Super Ikonta as it appeared in ebay
Sometimes my photographic investigations wander slightly afield from my focus on Nikon and Tenax cameras. Here we have a Zeiss Ikon Super Ikonta A (Bestell #530) from the mid-1930s. It is of the second type with the shutter release moved onto the body, but before the addition of lots of chrome that marked the 531 versions.

It is equipped with a 7cm f3.5 Tessar in a Compur shutter. At the time it was sold, this was the top of the line in 4.5X6cm 120 size roll film cameras. The rangefinder uses a rotating wedge mechanism coupled to the front element of the lens. These cameras were not cheap and even today Super Ikontas in good condition generally sell for well over $100 on ebay.

The 4.5 X 6cm format has a certain appeal to me. Large enough to be medium format, but still small enough to allow compact lenses and a compact camera body. The extra frames on a roll of 120 film is a nice plus.
    So when I spotted this Super Ikonta A on ebay for only $50.00 (US) I took note. As you can see in the first picture, the condition was not promising, but I decided to take a chance and bought it.
    Several hours of labor using saddle soap, leather restorer, a soft toothbrush, toothpicks, Q-tips, lens cleaner, lens tissue and soft cloths and I had what you see here. Oddly enough, it came with two 620 spools, which I promptly sold on ebay.

    Bellows is still light-tight. Shutter is reasonably accurate, lens is clear. Everything as
works as well, except the frames of the albada finder are badly faded and the rangefinder is dim. Still a great picture taker. And results? Nice, sharp pictures with plenty of contrast and depth.
Quechee Gorge, Vermont
Scott and Charlotte at Quechee Gorge

Friday, May 11, 2018

Zeiss' 50cm f8 Fernobjektiv

The longest lens that Carl Zeiss in Jena manufactured for 35mm cameras was a 500mm f8 lens it referred as a "Fernobjektiv" or Far lens. Part of the text below first appeared in an article I wrote for the 2014 Spring/fall issue of Zeiss Historica [Vol 36, no.1]
50cm f8 Fernobjektiv on a postwar Panflex and a Nikon SP.

In the late 1940s, as Zeiss was trying to figure out what company it still was and what market it should aim for, the Jena factory completed a run of 40 Fernobjektivs, supposedly all in the post-war Flektoskop mount (#s 3412641—3412680, order #907, 8 August to 15 November, 1948). When the Jena facilities finally got around to some kind of regular production, new Fernobjektivs came out in mounts for the East German Zeiss Flektometer housing or with adapters for Exakta or in the M42 thread. Production continued into the early 1960s.

This Fernobjektiv uses the external bayonet mount. It works equally well on a BR-1 ring.
So how did the 50cm Fernobjektiv pictured here end up in the Panflex mount? The serial number  (#3412655) is within the same 1948 batch that were supposed to be in Flektoskop mount. So it must have been intended for the pre-war Panflex. Were a few of that batch made and sold in the Panflex mounting?

The lens itself is a single component achromat doublet in a long-focus design. Such a simple design is capable of excellent sharpness and contrast particularly in the center of its field—provided the lens is confined to a modest aperture and narrow coverage. The famous long-focus lenses offered by Novoflex use a similar optical design. The finish of this particular lens is good if not spectacular. A nice modern black crinkle finish was applied over a smooth black coating with the exception of the diaphragm and focus rings. The glass is T-coated. It takes readily-available 77mm accessories. Apertures close down to f45 with a manually-controlled iris mounted nearly 16cm (seven inches) behind the glass. The focusing helicoid is set forward and just behind the glass. The lens focuses as close as six meters (19.1 feet) with a nearly 360 degree turn of the focus ring. Only the glass moves; the diaphragm remains stationary.

Mounted on a Panflex with a Contax or Nikon rangefinder camera attached, the lens balances on its rotatable tripod bushing. Weighting only 1.627 kilos (4.5 pounds), the lens should be handholdable. However, the focusing ring is set so far forward and has such a long turning that focusing while trying to hold a camera steady becomes an exercise in frustration. This is particularly true if one is trying to follow a moving object.

Using a BR-1 ring, this lens mounts on any F mount Nikon, requiring on a slight shift of the focus to bring infinity into sharpness.
Like all lenses with focal lengths longer than 250mm, the Fernibjektiv vignettes on a reflex housing. This is due to the small throat of the Contax/Nikon bayonet system.
On a Panflex, the Fernobjektiv vignettes.

However, on a Nikon F, it does just fine and is capable of nice, sharp results.

Immature female Coopers Hawk Accipiter cooperii at our birdfeeder. It did not come for the seeds.

Tuesday, January 23, 2018

Mountain photography

Someone asked what are the best lenses to use for mountain photography. My answer: any focal length you want. Here are a set of examples taken the summer of 1972. Linda and I hiked to the summit of Sheep Mountain in the Gros Ventre Range of Northwestern Wyomissing.
All taken with Kodachrome.
Linda and myself on the summit of Sheep Mountain. Nikon SP and 28mm W-Nikkor. Note Nikon F in my lap.

Jackson Hole looking Northwest from Sheep Mountain. The Teton Range across the valley. Nikon SP 28mm W-Nikkor.

Middle Teton, Grand Teton and Mount Owen from Sheep Mountain. Same view as 28mm W-Nikkor but using 400mm Tele-Nikkor on Nikon F.

Monday, September 25, 2017

Searching for the Tenax III

Page 1 of a British patent application from 1936
It has been known since the 1980s that Zeiss Ikon intended to produce a Tenax III with a built-in meter similar in principle to the Contax III.

Larry Gubas’ presentation of a patent drawing on Facebook for a light meter contained within a camera body is evidence of the changes Zeiss Ikon was considering for its various 35mm cameras. What I find interesting is that the camera type that appears in the drawings is definitely a Tenax of the Tenax II type, and not a Contax. This suggests that Zeiss Ikon intended to build the same metering system into both its Contax and its Tenax.

The evidence in the drawings: the proportions of the body, the placement of the rangefinder windows and the square viewfinder window are all pure Tenax. The shape of the lens and placement of shutter and focus levers are also Tenax. There is no way that this camera could be a Contax. But what about the famous plunger advance? The placement of the proposed meter window is certainly in the way.

For some time, a Tenax II protoype has been known in which the plunger  advance has been replaced by a horizontal lever that projects from under the frame counter. It was once thought to be a prototype for the military, but with the understanding that a meter window on a Tenax III would require the repositioning of the film advance, this camera becomes evidence of how Zeiss Ikon might have solved the problem of a lever blocking a meter window. This patent drawing also is the first evidence I have seen as to what the Tenax III might have looked like had it been manufactured. While the advance would have had to be relocated, the height of the camera (and therefore, the accessories needed) would have remained the same. The camera would have remained compact and fast operation and ease-of-use would have been unaffected.
The Tenax II prototype. Picture courtesy of S. Baumgartner

The Contax IV itself exists as a several drawings plus a number of incomplete prototypes which Gubas has shared in his book and on line. It was to be a revolutionary rather than an evolutionary product retaining only Heinz Küppenbender's vertical shutter and the external bayonet of the Contax II and III. Why?

The answer lies in patents, corporate politics and the limitations of 1930s technology. The Contax’s vertical shutter was complex and took a lot of space “under-the-hood.” Yet Küppenbender considered it his masterpiece and would not allow its abandonment. What little room was left inside was taken up by the complex internal/external lens mounting system with its built-in focusing mount, gears, sliding focusing prism and the huge bar prism that ran the width of a Contax. To maintain all this, Hubert Nerwin and his design team had to place the meter outside and on top on the Contax III. While a workable solution, it raised the height of the camera and added bulk and weight to a camera already heavier than its Leica competition. It also meant that any finders and other accessories would no longer line up in the same way as ones for the shorter Contax II. This is why there was a separate Contameter  (1340) for the Contax III, as well as different masks for 85 and 135mm lenses.

The Contax IV was an attempt to solve all these problems in one complete redesign. By eliminating the built-in rangefinder system and the internal lens focusing mount, Nerwin was able to gain enough room to move the meter inside the camera, eliminating the meter hump and duplication of accessories. This also meant that all lenses would use the same mount and handle in the same way—a design imperative for Nerwin. The focusing lever and external rotating prism system that the Contax IV advocated was the same as used on the Nettax and had already proved itself on the Super Ikonta series of cameras. It promised the same focusing accuracy as the Contax, but allowed faster focusing via a lever instead of a slow-turning wheel. By continuing the use of the external bayonet mount, all external accessories would remain the same, and legacy lenses would still mount, although focusing would be by guess.

The placement of the meter window below the film advance may seem a bit strange, but it was to be in the camera but away from the rangefinder window. The left hand would now be doing the focusing for all lenses—the same as the Nettax and the Tenax II—and the right hand would remain on top of the camera for winding and shooting. All-in-all, a workable design for both the Contax and the Tenax that would fit into Nerwin’s design priorities for ergonomics and simple, intuitive handling.

Sunday, July 2, 2017

The 2.7cm f4.5 Orthometar, Zeiss Ikon's rare wide angle for the Tenax II

The 2.7cm Orthometar lens for the Tenax II. Note that this particular lens is marked in feet. It dates to October 1937, part of a patch of 100 built before the Tenax II camera was released for sale in April 1938.

Hubert Nerwin designed Zeiss Ikon's Tenax II camera of 1937 as a fast-action rangefinder camera with fully interchangeable lenses. That meant its lens offerings had to include at least one wide angle. This proved to be a bit of a challenge. There were few options in wide-angle designs in the 1930s. Initially the Contax had made do with a slow 2.8cm f8 Tessar that did not even couple to the rangefinder. The later fast and sharp 3.5cm f2.8 Biogon was the best wide-angle that any lens manufacturer could offer at that time, but its depth precluded its use on the Tenax. So Nerwin chose  an Orthometar design.
    The Orthometar was designed as an aerial lens by Dr. Robert Richter. As its name implies, it was particularly free of distortion—an important consideration in a lens used for aerial mapping. A modified double Gauss design, it appears similar to the 28mm f3.5 W-Nikkor for the Nikon RF cameras except for a much smaller back element.
    Zeiss also took the design and used it in a 3.5cm f4.5 lens for the Contax cameras—slower than the Biogon, but priced at a lower price point than the Biogon.
   Adapted for use on a 24X24 format camera, such  as the Tenax II, the 27mm focal length yields the same angle of view as a 34mm lens on a 24X36mm camera.

    Although it takes the same 34.5mm screw-in filters as the other lenses for the Tenax, the Orthometar optics are small, a mere 6mm across the front. Grinding the inner elements must have been a difficult and challenging task. The five aperture blades are also tiny. The spaces between aperture markings are even and the aperture closes down to f32. The barrel does not rotate when focused. This is a modern lens lacking only clickstops to make it comparable to any RF lens made today. It focuses down to less than three feet with a 30 degree swing of its under lens focusing lever.

Carl Zeiss made only three lots of this lens: two prototypes in 1937 (nos 2025791 and 2025792), then a production run of 100 finished on October 1st, 1937 (nos.2203301-2203400). A final run of 200  probably came close to a year later (nos 2360001-2360200). The lens was expensive ($132 dollars, or equivalent to over $2200 at 2017 rates) and the market for wide-angles limited. The outbreak of World War II less than a year-and-a-half after the camera’s introduction undoubtedly contributed to the low numbers and small sales.

With such a small production, it is a wonder that this lens is available at all, but I snagged the one illustrated here off ebay with a buy-right-now option complete with its finder and a Tenax camera and custom case. Unfortunately, the camera had been dropped, but with parts on hand I have been able to get everything back in working order.

The lens is easy to use and the results sharp. I like its compactness and easy to set controls.
More details on the Orthometar for the Tenax can be found in my book on the Tenax II, available from Blurb Books:
Orthometar mounted on its original Tenax with finder 482/6

Orthometar images