It’s been three years since we announced our ’50mm f/2 Bertele’ project. We hoped to start accepting pre-orders as early as 2021, but life (and business) doesn’t always follow a straight path. The original design has been revised multiple times after consulting Sonnar experts and enthusiasts to arrive at the best final product we could deliver. The manufacturing process took much longer than we expected.

Today, we finally have a set of fully completed pre-production Bertele lenses. It feels like a big moment because Bertele is no longer just a concept on a piece of paper but a physical, real, fully working, and gorgeous piece of photography equipment (if we say so ourselves). The next step is sending the prototypes to various lens experts for testing, reviews, and feedback.

THE FORMULA:

The Bertele 5cm f/2 MC lens is an optical remake of the 1934 Sonnar formula first designed by Dr. Ludwig Bertele for Carl Zeiss Jena.

The Bertele lens combines this classic 90-year-old lens design first seen on the Nickel lenses of this era, with sapphire blue and violet double coating first seen in the post-war era lenses. The mechanical lens body is solid brass with a native M bayonet. The lenses can focus from 0.7m to Infinity and are fully rangefinder coupled.

They use a new type of mechanical lens design (a non-linear pre-loaded reverse RF cam slope technique) that helps address the lenses’ focus shift.
The original 5cm f/2 Sonnar formula was a significant evolution of lens design when it was released, so bringing a new generation in contact with this vintage formula for usage in today’s world was the Bertele lens’s primary goal. That, and most of us involved with the project, are also massive Sonnar fans, so we were creating our own dream Sonnar.

We found that the original 5cm f/2 Sonnar formula combined the aesthetically pleasing bokeh/ rendering with an image that, when stopped down, is sharper in the corners than the Opton versions can achieve. Thus, while it often gets overlooked in favor of either faster or later models, at least for us, the 1934 5cm f/2 formula has the nicest balance of smooth bokeh and in-focus sharpness.

The Bertele optics render what can only be described as a warmer, sharper, more color-saturated, higher contrast image than its predecessors, even though we still maintain the usage of the classic double coating.

We will release more information about the optics in future updates.

FOCUS SHIFT TAMED

Permitting a 50mm Sonnar formula lens to be focused accurately across the entire focusing range (0.7m to Infinity) was one of the priorities during Bertele’s development. The non-linear pre-loaded reverse RF cam slope became our answer to the age-old problem of closer-distance focus shift.

Wait, the Bertele lens can mechanically deal with the Sonnar formula’s secondary focus shift?! Well…Yes!

The new design implements a pre-loaded reverse RF cam slope into the lens body that accurately translates the Sonnar formula’s native 52.4mm EFL to 51.6mm EFL, which the Leica camera expects to receive at the swing arm. When the lens goes below 1m, this slope can become non-linear because it’s rotating when focusing (something the LTM versions never did). Because the M bayonet always orientates the lens correctly on the M camera, misalignment for the non-linear slope becomes impossible.

The type of RF cam slope we use on Bertele is a new design that permits multiple EFL value translations at different distances. Unlike linear RF cams that only translate a single EFL value, the RF cam of Bertele can translate 52.4mm to 51.6mm at the 1m to Infinity focus distances. As the lens focuses closer, the rotating RF cam slope enters into a non-linear 52.5:51.6 translation, followed by a 52.6:51.6 translation, etc. At Skyllaney, we refer to the feature as a fork-tail, which we must implement on the RF cams of challenging lens conversions with optical EFL values that shift below 1m focus distance (like Kinoptik 75mm and Angeniuex 90mm lenses when close focusing).

The other aperture-dependent focus shift we addressed in a different way.
We chose to calibrate the Bertele lenses for wide-open f/2 usage. From servicing hundreds of Sonnars for clients over the years, 99% of the users want the lenses wide-open optimized. With f/4 and f/5.6 being the two aperture-dependent focus shift areas, when f/2 is the calibration aperture, they became painted red on the Bertele lens to highlight to the user these apertures deviate from the RF focus patch.

With Bertele, we also ensured each optical block was 52.4mm EFL with minimal deviation. Unlike the usual acceptable range of 51.8mm through 53mm (+/-0.6mm) EFL, we close this tolerance to almost zero. This extra attention to ensure the optical block is highly EFL adjusted and calibrated permits the non-linear pre-loaded reverse RF cam slope to function correctly because we know what EFL value the lens formula is then shifting to at what distances, and this remains constant across all units.

For the first time on a 50mm Sonnar formula design, made specifically for Leica cameras, the closer focus range can finally be unlocked without fear of the RF focus being wildly off in these areas due to outer element aberration heightening. Thus, with the new mechanical design, the 1934 Bertele formula can now focus accurately down to 0.7m.

MEDIUM FORMAT

Optically, the Bertele lens, based on the 1934 Sonnar formula, has the same clear open aperture trick up its sleeve as its paternal Nickel variant. It can cover digital medium format (GFX/ CFV).
Bertele maintains this digital medium format capability, yielding an image circle exceeding 45mm in diameter, which adequately covers the 44mmx33m digital medium format sensors.

THE MANUFACTURING

Our machine shop at Skyllaney is set up only for small volume work making custom parts for one-off or small batch conversions.

In order to handle manufacturing hundreds of parts a larger machine shop was required to scale up production. Cooke and Taylor Hobson made rangefinder lenses in the UK until the 1970s. Unfortunately, much of the mechanical manufacturing know-how had been lost since then and had to be rediscovered. Fortunately, we found a machine shop in Warwickshire, England, willing to learn and innovate, but it took a considerable time to arrive at full mechanical rangefinder fabrication abilities. We invested significant money into the company to get them up to the level of making mechanical lens helicoid components.

One such machine required was the Doosan Puma, a special CNC lathe with a situationally aware headstock with micron gradient Mitutoyo glass scales. This machine is used to manufacture the inner and outer brass helicoids to have a controlled 35-micron gap between mating parts, which we found perfect for permitting a thin layer of grease. The Doosan Puma gives us high axial concentricity with minimum play and extremely smooth focus operation on the lens components we make. It’s just one of the many new advancements in helicoid manufacturing we had to develop and refine when making the Bertele lenses.
Once all the pre-production parts were made, and quality checked, the bodies were sent for plating and finishing, followed by a final assembly at Skyllaney, in Scotland.

The process of laying down manufacturing capability and refining it to make rangefinder lenses again in the UK took most of the last three years. During that process, we developed a sister rangefinder lens line known as Omnar Lenses with our partner Hamish Gill of 35mmc. These two lens lines, Bertele and Omnar Lenses, are now merging into one.

OMNAR LENSES UK

We intend to have the Bertele lens join the other lens offerings we manufacture for Omnar Lenses, all under one banner going forward. Thus, the Skyllaney 5cm f/2 MC Bertele is now by Omnar Lenses and will be promoted and branded as such. It joins the ranks alongside the CN26-6, NK35-25, CT38-28, LT42-45, and recently announced NK35-18 lenses.

FIRST REVIEWS

Brian Sweeney, whom we consider the Sonnar guru and mentor, was very much involved in the mechanical development of the lens. He reviewed our mechanical drawings and advised us on the pitfalls of the older LTM Sonnar lens designs he had encountered over his decades of dissecting those lenses. The current design is as much a collaboration of what his dream Sonnar would be like as ours.

Brian Sweeney has been beta-testing a pre-production copy for the last few months and is the first to provide feedback on this lens’s performance. You can access it by clicking the link below:

https://rangefinderforum.com/threads/ninety-years-of-sonnars-from-the-carl-zeiss-jena-5cm-f2-to-the-the-5cm-f2-bertele-sonnar.4815091/

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