vintage lens

Vintage lens makers − Novoflex (Germany)

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Novoflex is a German maker of lenses and camera accessories (macro bellows, tripods, tilt-shift bellows, etc.). It was founded in 1948 by photographer Karl Müller Jr. In 1949 the company produced the reflex housings for Leica, which allowed SLR lenses to be modified for use on Leica cameras. These were initially marketed under the name Reproflex, until being changed to Novoflex in 1950. From 1954 housings were also made for Contax cameras.

Fig.1: The basic Novoflex Follow-Focus lens system

In 1956 they started production of their first lenses, the Novoflex Follow Focus lenses. The Follow Focus lens system was interesting because it included a pistol-grip focusing device that allows the user to go from infinity to minimum focus in a split second. Essentially it provides one-handed focusing. According to the company this was useful for “wildlife subjects in full flight, sports, the fleeting moment, the unexpected are unusual picture opportunities that must be taken at peak-action.”

Fig.2: The telescoping lens and Noflexar

This was followed in 1960 with nesting telephoto lenses, advertised as ‘telescopic tele lenses’. These were designed in order to make telephoto lenses easier to transport, being able to collapse to half their size. The focusing unit could be equipped with lens heads for 400mm and 640mm. In 1962 the company introduced the ‘2-in-1 lens’, 35mm f/3.5 Noflexar, a macro wide-angle lens with a focusing range from infinity to 2.75”, and a reproduction ratio of 1:2. In 1969 the company started making automatic bellows devices. The company had an extensive range of ancillary products for many camera systems. This included a wide-angle macro lenses, bellow units, follow-focus lenses, slide copiers, and associated coupling adapters.

Fig.3: Vintage ads for Novoflex lenses

Novoflex is still an active company, producing photographic accessories such as auto-bellows, tripods, macro systems and camera-lens adapters.

Vintage lens makers − Kinoptik (France)

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Kinoptik was founded in Paris in 1932 by Georges Grosset and Georges Perthuis. Grosset began by creating 35mm camera optics with a series of Apochromat lenses in 1939 (lenses with better correction of chromatic and spherical aberrations), all with the same double-gauss optical structure.

The workshop was destroyed by the RAF in March 1942. Production resumed in the summer, however during the German occupation they were forced to produce Askania camera sights. However this didn’t stop Grosset from designing two new lenses, the Fulgior 50mm f/1.3 (which was used on the Rectaflex), and the Apochromat-C 32mm f/2.8. Postwar, French cinema boomed and Kinoptik concentrated its efforts on the cinema business. It designed numerous lenses for 16mm, Super-16 and 35mm cine cameras.

In the early 1950s they also diversified into optical systems for microfilm, medical radiology, and control of industrial furnaces. The company bore the Japanese competition in the 1960s better than most of its European counterparts due to its business in professional cinema equipment. From a 35mm perspective, Kinoptik produced a number of lenses for ALPA, as well as Leica, Nikon, Canon and Minolta. Examples include the Apochromat 100mm f/2 and the Aplanat 500mm f/5.6. The Apochromat 100mm f/2 was described as having exclusive correction of all primary colours, critical sharpness and highest contrast, even at full aperture.

Fig.1: Various Kinoptik lenses

After the death of Georges Grosset, his wife Marie-Louise Grosset took over running of the company, and hired French optician Edgar Hugues (1915-2004) who became technical director of the company from 1957-1964. He designed the 75mm f/1.1, 100mm f/1.3 as well as the Lynxar 60mm f/0.7, arguably the fastest French lens ever created. He also designed the Tegea rectilinear “fish-eye” lenses (130° angle-of-view for 24×36mm). One such lens, the Tegea 9.8mm f/1.8 was used by the likes of Stanley Kubrick in films like A Clockwork Orange (1971) and The Shining (1980).

In 1981 the company was sold to Société de Fabrication d’Instruments de Mesure, after which it underwent numerous integrations, mergers, and acquisitions before closing in 2003. The lenses were by no means inexpensive. In 1980 prices, the 50mm f/2 Macro-Apochromat sold for US$999, and the 100mm f/2 Auto-Apochromat for US$799. Vintage Kinoptik lenses are still vogue in the film industry, often rehoused in new bodies. The Apochromat 100mm f/2 sells for anywhere from US$5000-7000 on todays market.

Where have all the lens names gone?

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There was a time when lens manufacturers gave their lenses names − the likes of Pancolar (Zeiss Jena), Noctilux (Leica), Biotar (Zeiss), Switar (Kern). In some cases the names were derived from Latin words, which were meant to describe some intrinsic characteristic of the lens. For example Leica’s Summilux. The combination of ‘summi’ (meaning ‘highest point’) and ‘lux’ (Latin for ‘light’) results in the intimation of ‘maximum light’ – referring to the enhanced light-gathering abilities of these lenses. The name appeared in 1959, and has been a staple ever since.

Fig.1: Various historic lens names

But in the modern era, few companies do this anymore. There seem to be three exceptions: Leica, Zeiss, and Voigtländer, although to be honest, most Japanese companies did not give names to individual lenses. Asahi was the exception with the Takumar line, although it signified a time period and technology more than the characteristics of an individual lens. Leica still use names for historical families of lenses. Some of this may be tradition, helping users to identify certain characteristics of a lens just by reading the name. Zeiss is another company that still names its family of lenses, e.g. Touit (a small Brazilian parrot). Giving lens families names makes things a lot simpler, and also provides a better brand association. That’s why people still remember Zeiss’s Pancolar lenses, or Asahi’s Takumars.

Names existed to invoke some sort of an emotion. Like giving a lens a name gave it a sense of power. It is more likely to be remembered than the modern trend of adding a string of incomprehensible hieroglyphs of abbreviations – something most people forget in quiet quickly. But once you have heard of the legend that is the Pancolar, you are unlikely to forget it. The likes of Pancolar and Flektogon bring memories of lenses with exceptional background blur (aka bokeh), from East Germany. There are of course the classics of German lens design: Zeiss’s Tessar, Planar, Sonnar, or Voigtländer’s Ultron.

Fig.2: What’s in a name? The Voigtländer Nokton

Perhaps because there are always new lenses, and it would be hard to keep up with new names? Perhaps in the information age, names have been supplanted by acronyms, and abbreviations? It’s no different to cameras, which no longer have names anymore either. The Ricoh GR series was (supposedly) named after two of the first cameras marketed by Riken Kōgaku Kōgyō, the predecessor of Ricoh, namely Gokoku, and Ricohl. But without digging for it, nobody actually seems to know what GR means.

Fig.3: Some naming ideas for TTArtisan’s 35mm f/0.95 lens

I would like to see lens manufacturers look at giving lenses actual meaningful names. Not all of them of course, but perhaps families of lenses. Perhaps a line of lenses named after prominent lens designers, like Willi Merté who designed the Biotar for Carl Zeiss in 1927. Or a fish-eye series named after physicist Robert W. Wood who coined the term ‘fisheye lens’ in 1906. TTArtisan has a few lenses with a vintage striped aesthetic, why not give them a name that pays homage to the 1960s lenses? Or perhaps a name associated with the abilities of the lens. Two name suggestions for the TTArtisan 35mm f/0.95 are shown in Figure 3. I’m not suggesting that coming up with viable names will be easy, but once established they will help strengthen brand association.

Vintage lens makers – Feinmess (Germany)

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Feinmess was founded by Gustav Heyde (1846-1930) in Dresden (1872) as Gustav Heyde – Mathematical-Mechanical Institute & Optical Precision Workshop (Feinmess roughly translates to “fine measurement”). The company produced astronomical and geodetic precision devices: binoculars, domes and refractors for observatories, telescopes, theodolites (land surveying devices), hand-held rangefinders for aerotopography, and actino-photometers (light meters). From 1931 the company was converted to a limited partnership and concentrated on arms production. In 1945 the company operated under the name Gustav Heyde GmbH. After the war the company was expropriated by the state of Saxony operating as Optik, VVB für feinmechanische und optische Geräte. Finally in 1948 it changed its name to Optik – Feinmess Dresden VEB.

In the 1950s, Welta (Freital) requested a lens for their Belfoca 1 and 2, medium format cameras. There was so much demand for lenses that Feinmess accepted the order, never having produced lenses before. The optical design office was set up by Ihagee, and work on the design of the lens was taken over by Claus Lieberwirth is August 1953. From 1954 the Bonotar was created as a 105mm, f/4.5 lens. A second lens, the 105mm f/6.5 Bonar was derived from the Bonotar. Both lenses were simple in construction, and inexpensive. About 20,000 M42 and 4,000 Exakta mount Bonotars were produced. The lens established itself as a cheaper alternative to the popular Meyer Optik Trioplan. In 1960 production of both lenses was halted, and the optics department was eventually merged into VEB Carl Zeiss Jena.

Interestingly, VEB Feinmess was used in the 1950s as “shield” company, especially for patent applications related to VEB Zeiss Ikon, due to the issues with Zeiss-Ikon Stuttgart. This is why camera patents for well known GDR products are the property of VEB Feinmess, until the founding of VEB Kamera-and-Kinowerk in 1959. There are literally hundreds of patents for lenses, viewfinder systems, motor winding systems, and viewfinder cameras (to name but a few) − all products that Feinmess did not manufacture.

The company still exists today, recently renamed from Feinmess Dresden GmbH to Steinmeyer Mechatronik GmbH, and makes various measuring instruments, positioning systems and optical devices. Bonotar 105mm lenses can be found for between €60-90.

Notable lenses: Bonotar 105mm f/4.5

What happened to the Zeiss lens collection?

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When Carl Zeiss Jena was still under US control in June 1945, the US Army Signal Corp’s Pictorial Division expropriated the “Zeiss lens collection”, which consisted of approximately 2000 sample lenses, and associated documentation. The collection was handed over to Colonel Tebov on May 12, 1945 in Jena.

The collection represented not only Zeiss lenses, but optics from other manufacturers, and was used in research and production control. The lenses were transferred to the Signal Corps laboratories at Fort Monmouth, and the documentation to Dayton-Wright Army Air Field in Ohio. At Fort Monmouth, chief of the photographic branch (Signal Corps Engineering Laboratories) Dr. Edward F. Kaprelian, studied the lenses, attempting to understand and recreate the optical designs in many of the prototype Zeiss lenses. Supposedly the lenses were to be analyzed, in particular several hundred experimental lenses that were never sold. None of these historically and technically significant lenses had been clearly documented as part of the appropriation. Willy Merté, head of optical computation at the former Carl Zeiss Jena was apparently languishing in a refugee camp in Heidenheim before Carl Zeiss could begin operation in Western Germany. Merté would go on to catalogue the collection.

In April 1947, Popular Photography was the first major US publication to give a two page sneak peek [1]. Example lenses described include:

  • The Spherogon, a 1.9cm f/8 lens with a plano (flat) front element 3” in diameter, with an AOV of almost 160°.
  • The R-Biotar, was the fastest commercially produced lens in the world, at 4.5cm with an aperture of f/0.85. It was used for 16mm movies of fluorescent x-ray screens.
  • The Bauart BLC, a 20cm f/6.3 objective used by the Luftwaffe for aerial mapping.
  • The Perimetar 2.5cm f/6.3 for 35mm cameras, covering a 90° AOV with a deeply concave front element.

Probably the best description of some of the more unusual lenses comes from a June 1947 article by Kaprelian himself [2]. In it he describes some of the V (versuch) or experimental lenses. He describes lenses like the V1940, a 7.5cm f/2.8 lens with a 70° AOV, with little astigmatism or coma, and very little in the way of distortion. Or the V1935, 10cm f/6.2 lens whose front element is strongly concave. Another lens already produced in certain quantities was the Sphaerogon, available in focal lengths from 1.6 to 12cm and f/7, f/8 apertures. Other lenses include experimental aspherical surfaces, telephoto, and wide-aperture lenses.

Where are these lenses today? Perhaps stuck in a storage locker somewhere in the vast storage facilities of the US Army? Well, actually no. In an article in Zeiss Historica in 2016, the fate of the collection is documented [3]. Stefan Baumgartner bought a number of lenses from the collection in 2006, and as he tells it, this is when a major portion of the collection was put up for sale on eBay, a legacy of the estate from American photographic businessman Burleigh Brooks. Apparently after Kaprelian’s release from his military service the collection was left in the custodianship of Burke and James in Chicago, occupying warehouse space for about 20 years. It was later disposed of as military surplus, which is why Brooks probably acquired some of the lenses (as he owned Burke and James).

Further reading:

  1. Walter Steinhard, “Lens Oddities”, Popular Photography, 20(4), pp.82-83 (1947)
  2. Edward K. Kaprelian, “Recent and Unusual German Lens Designs”, Journal of the Optical Society of America, 37(6), pp.446-471 (June, 1947)
  3. Stefan Baumgartner, “A Mystery of Another Lens from the Zeiss Collection”, Zeiss Historica, 38(1), pp.17- (Spring, 2016)

Ultrafast lenses – the Zunow 5cm f/1.1 (and 58mm f/1.2)

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The 1950s photographic industry in Japan was marked by a race to develop the fastest lens. In December 1950, Nippon Kogaku, maker of the Nikon, would introduce the Nikkor 50mm f/1.4, the fastest normal lens produced. But the victory was short lived, as in 1953 an even faster f/1.1 lens was introduced by a little known company – Zunow.

Fig.1: Literature in Japanese news announcing the f/1.1 lens

The development of the Zunow 5cm f/1.1 began in 1943 at the Teikoku Optical Co. to meet the high-speed optical needs of the Japanese Navy [2]. There was a requirement for a fast lens in low-level light situations such as aerial surveillance at dawn and dusk. The design was spearheaded by Sakuo Suzuki, and Michisaburo Hamano (NY Times, Nov.21 1953), and managed to produce three prototypes, but the factory was destroyed in a raid late in the war. It would take ten years to complete the lens. The first prototypes were completed in 1950, and the 50mm f/1.1 was Zunow released in 1953.

Fig.2: Lens diagram from the patent with associated optical glass types

The patent for the Zunow f/1.1 lens [3] describes the lens as “an improved photographic objective suited for use with a camera that takes 36×24mm pictures”. The lens had a configuration of 9 elements in 5 groups, in a Sonnar-type design, likely derived from the Sonnar 50mm f/1.5. It was available in mounts for Contax, Nikon and Leica rangefinder cameras. The amazing thing about this lens is the fact that it was not constructed using any “rare earth” glasses.

Fig.3: Comparison of lens diagrams for Sonnar f/1.5, and the two versions of the Zunow f/1.1 (hatched lines indicate new glass) [2]

The original version earned the nickname “Ping-Pong ball” because it featured a rounded end. However it faced some issues, mostly when the aperture was wide open, e.g flare. Kenji Kunitomo and Yoshitatsu Fujioka would join the company to address these issues. This lead to the introduction of the Type 2 in 1955. The new design dealt with the protruding ball structure by redesigning the lens. It was transformed into a flat rear design with 8 elements in 5 groups, which also dealt with the flare and brightness issues when wide open. The lens elements featured a hardcoating on all air-glass surfaces to reduce internal reflections [7].

Fig.4: Specs for the original 1953 lens

Comments on the lens performance in The Truth About Superspeed Lenses (1957) [4]:

  • Performance: Vignetting at the widest aperture f/1.1, disappears completely at f/2.8. The lens is acceptably sharp at the centre of the negative. Detail is lost toward the edges and corners. Sharpest range is f/5.6 to f/11.
  • Comments: The Zunow lens mount may cut off the right corner of some camera viewfinder windows, blocking part of the image. It might be a good idea to use an accessory viewfinder with the Zunow. We found it an easy lens to focus quickly.
Fig.5: The “Ping-Pong ball” lens

While the Zunow 50mm f/1.1 lens was the first ultrafast lens for rangefinders, there were few if any lenses of equal stature in the SLR realm. Ads at the same time showed a Zunow 58mm f/1.2 fast lens for Exakta and Pentacon SLR cameras. While very few have seen this lens in real life, and it does not appear to have been sold at any auctions in recent memory, there is a glimpse into what it looked like in the one of the ads for the Zunow camera shown in Figure 6. It was apparently a 7 element/5 group lens, of some expanded double-Gauss design [6]. The 58mm f/1.2 would have been the fastest lens offered by any camera manufacturer at that time for an SLR. In all likely so few were made that they today sit in private collections.

Fig.6: An ad for the 50mm f/1.1 lens, and a sneak peak at the 58mm f/1.2 in a Japanese ad for the Zunow SLR

Nevertheless, it would take Nippon Kogaku until early 1956 to match the Zunow lens in speed, introducing the Nikkor 50mm 1.1. Canon was not in the picture until the 50mm f/0.95 in 1961, and Leitz not until 1976 with the Noctalux 50mm f/1.0. The Zunow 50mm f/1.1 is today a vary rare lens. Sales are are US$5-10K, up to US$20K depending on condition, and mount. The price for this lens in 1956 was US$450, although it could be found for as low as US$300.

Fig.7: The Zunow SLR showing the 58mm f/1.2 lens, and its 7/5 configuration

Further reading:

  1. Norman Rothschild, “Meet the Zunow f/1.1”, Popular Photography, pp.126/128, February (1956)
  2. Kogoro Yamada, “Japanese Photographic Objectives for use with 35mm Cameras”, Photographic Science and Engineering, 2(1), pp.6-13 (1958)
  3. U.S. Patent 2,715,354, Sakuta Suzuki et al., “Photographic Objective with Wide Relative Aperture”, August 16, (1955)
  4. “The Truth About Superspeed Lenses”, Popular Photography, 21(10) pp.62-64 (1957)
  5. Zunow Teikoku Kogaku Japan 50mm f1.1 – The First Ultra Fast Lens
  6. Tsuneo Baba, “Zunow: Indication of things to come in 35mm single-lens reflexes?”, Modern Photography, 23(4), p.110 (1959)
  7. “New f/1.1, 50mm Zunow Lens”, Popular Photography, 20(5) pp.26,30 (1956)
Fig.8: The 50mm f/1.1 lens in various guises

Superfast lenses – the Taika Harigon 58mm f/1.2

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Not all fast lenses came from the lens giants. Other slipped under the radar. A good example is the Harigon 58mm f/1.2. It was made by Taika, which in reality was a export brand of Taisei Kōgaku Kōgyō K.K., the company which would later become Tamron (in all likelihood, Taika was a simpler and easier brand to remember than the company name). It was also sold as a Tamron lens.

The Harigon 58mm f/1.2

The lens was introduced in 1960, for the US market. The design was likely along the lines of the Zunow 5.8mm f/1.2, which is a itself is scarce as hen’s teeth.

Optical Science has produced this all-new Taika Rare Earth Lens, Eight hard coated complex elements of fabulous precision are responsible for its superb color correction and sharpness beyond reproach. The Taika Harigon has a dignified beauty – ebony black with colored engraved scales. A “Rolls Royce” in performance and appearance. You will be proud to own it, proud to show it and proud to exhibit its beautiful pictures.

Apparently it was available in Exakta mount, in addition to Praktica M42, and Miranda. In the early 1960s it was advertised as the standard lens on the Exa II camera for US$198.50 (from Seymour’s, NY). Interestingly the lens itself sold for US$169.50. By the time the Adapt-A-Matic Lenses appeared in the late 1960s, the 58mm lens had disappeared. There seem to be very few on the market today.

Further reading:

Vintage lenses – a 360° lens from the 1970s

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There aren’t many Swiss companies that manufacturer lenses apart from Kern, but one lens exists in the form of Volpi AG, a company based in Urdorf near Zurich. The company specialized in higher-end projection systems. In the early 1970s the company produced a lens called the Peri-Apollar 360°.

The Peri-Apollar 360°, nicknamed the “optical bell”, or “Swissorama” lens, does not use the fisheye principle or any of the other well-known panorama methods. It was developed by H. Brachvogel of Volpi AG, allowing the capture of 360° seamlessly in one image. If the camera is pointed with the lens in a vertical position, then the camera and photographer are covered by the centre of the image, which is blocked out. The inner edge of the circular ring is the lower edge of the image, the outer edge is the upper edge of the image.

Fig.1: The Volpi Peri-Apollar lens

The lens came in two focal lengths 25mm f/4, and 40mm f/5.6. The lenses could be adapted to many differing formats, including 16mm, 35mm, and 120 film (and could also be used as a periscope without a camera). The lens covers a complete circular image of 360°, without any gaps. When mounted in vertical position, the field of view has an angle of 60°, i.e. 30° above, and 30° below the horizon. The image is created according to the rules of central projection, where all verticals in the object field converge in a radially symmetric manner in the centre of the image. The lens was not actually intended for taking pictures in the horizontal direction

Fig.2: One of the few images available from the lens (somewhere in the Alps) Note that the point from which the image is taken, i.e. the lens itself in the image centre, is blacked out.

The light enters the protruding glass dome (which is an aspherical lens), and is refracted inwards at the transition between air and glass, and then totally reflected at the opposite glass-air interface. In this way the ring-shaped image is created in the front glass body. It then passes through a corrective lens and is projected onto the film by a lens of normal construction. Distance and f/stops can be set as with any normal lens.

Fig.3: A schematic of the lens configuration of the 40mm f/5.6 lens

The marketing material for the lens suggested applications in numerous fields, industrial applications, e.g. remote observation of pipes, police and military applications, recording of traffic intersections, aviation, and internal observation of nuclear reactors. The 25mm f/4 C-mount lens (with an attachable 90° periscope viewfinder sold for US$4,995 (1983); and the ALPA 40mm f/5.6 was US$3,595 (1977). In the US it was marketed by Karl Heitz. The lens is often attributed to Kinoptic because it appeared in their catalogs, however they did not produce the lens.

Lens specifications:

24mm×36mm16mm film
lens40mm f/5.625mm f/4
focal length of the peri-lens20mm15mm
aperture of the peri-lensf/1f/1
outer diameter of the image23mm11mm
inner diameter of the image8mm4mm
degrees, horizontal360°360°
degrees, vertical2×30°2×30°
number of lens elements84
aperture rangef/5.6 to f/22f/4 to f/22
close focusing distance0.1m0.1m
weight900g750g

Acknowledgement:
Data for the table, and help with schematics adapted from information in “Fisheye-Objektive und verwandte Abbildungstechniken (IV)”, J. Scheibel in PHOTO-TECHNIK UND WIRTSCHAFT, No.8, pp. 225-227 (1973)

Optical Anomalies – Are air bubbles problematic?

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Vintage lenses of a certain era often contain air bubbles, but this by no means suggests that they are of inferior quality. A 1940 article in Minicam Photography describes this as a fallacy [1]. It seems that in early cameras, some photographers may have been weary of such imperfections. In all likelihood there are like-minded individuals today.

“They may look like undesirable blemishes, but they are much more apparent visually than photographically.” [1]

In early glass manufacturing, air bubbles were practically impossible to eliminate. At the time the rationale provided was that bubbles formed when ingredients were melted together at temperatures of 2750°F to form glass. Even first-glass lenses contained some number of bubbles.

“In the manufacture of the famous Jena glass the various elements used must be heated for a given length of time and to a certain degree, the process being stopped at just the right moment whether all the air has been driven out or not. There is no alternative.” [2]

The article goes on to provide an example of a 6-inch, f/4.5 lens with a diameter of 32mm across the front lens [1]. They count 12 bubbles, on average 0.1mm in diameter. The lens has an area of 804mm², and the bubbles an area of 0.0942mm², making up 0.012% of the surface area. So only 0.012% of the light passing through the lens is impeded by the air bubbles. The outcome? Light interference caused by bubbles is negligible.

“The actual loss of light is inappreciable, and the presence of these bubbles, even if near the surface, has no effect whatever on the optical quality of the image.” [2]

“Air bubbles will be found in most high-class lenses and are a sign of quality rather than a defect, since at present it is impossible to make certain optical glasses absolutely bubble-free; their presence doesn’t affect the quality of the image in any way. [3]

In the literature for many modern optical glass manufacturers, e.g. Schott, there are caveats on bubbles (and inclusions). Basically bubbles in glass cannot be avoided due to complicated glass compositions and manufacturing processes. The melting of raw materials produces reactions which invariably form gas bubbles in the melt (typically carbonates or hydrogen-carbonates) [4]. These bubbles are removed in the refining process, when the temperature of the glass is increased, reducing the viscosity of the glass and allowing bubbles to move up through the melt and disappear. Some residual bubbles are still left from imperfect refining. However, it is actually quite rare to see bubbles in modern lenses.

So do they make a difference in vintage glass? According to much of the literature, not at all. Besides, vintage lenses are all about character – nobody is looking for a perfect image.

Further reading:

  1. “Fallacy: That “air” bubbles in a lens are a sign of inferior quality”, Minicam Photography, 3(8), pp.30-31 (1940)
  2. “The Crucible – Air-Bubbles in Lenses”, Photo-Era, 31(6) p.319 (1913)
  3. “Andreas Feininger on Lenses at Work”, Popular Photography, 18(3) p.124 (1946)
  4. TIE-28: Bubbles and Inclusions in Optical Glass, Schott Technical Information (2016)
  5. “The Impact of Air Bubbles in the Optics of Old Lenses”, Jordi Fradera (2020)