Funleader Contax Biogon 28mm f/2.8 Conversion

Funleader Contax Biogon 28mm f/2.8 Conversion

The Funleader Contax Biogon 28mm f/2.8 Conversion is a non-destructive Leica M-mount rehousing of the Carl Zeiss Biogon T* 28mm f/2.8 originally produced for the Contax G autofocus rangefinder system. Introduced in 2026 as part of the conversion programme developed by Funleader and Mr Ding Studio, it transfers the original Contax G28 optical assembly into a dedicated manual-focus helicoid while preserving the donor optics without permanent modification.

The conversion completes Funleader's principal Contax G conversion series alongside the earlier G35 and G45 projects. The first G35 conversion appeared in 2019, followed by the G45 in 2021 and a redesigned G35 V2.0. The G28 conversion extended the same concept to the wide-angle Biogon in 2026.

The retained Carl Zeiss optical assembly consists of seven elements in five groups. Zeiss specifies an actual focal length of approximately 28.5mm, a maximum aperture of f/2.8, a minimum aperture of f/22 and a 74-degree diagonal field of view on the 24 × 36mm format. The original optical assembly uses a six-blade diaphragm and accepts 46mm filters.

The Funleader conversion does not alter this optical formula. Instead, the original G28 lens group is removed from its Contax autofocus housing and installed in a dedicated Leica M focusing assembly.

The conversion housing uses a hybrid material construction. The exterior is aluminum to reduce overall mass, while brass is retained internally in the focusing structure. The jointly developed G28 conversion system has a documented weight of approximately 113g for the conversion kit alone and approximately 168g with the original G28 optical assembly installed.

Mechanical rangefinder coupling is provided for Leica M cameras. Funleader specifically describes the converted G28 as supporting rangefinder-coupled focusing across Leica M bodies.

The conversion is reversible. The original Contax optical assembly is not permanently modified during installation and can subsequently be returned to its original Contax G housing.

Reliable manufacturer specifications for the overall external diameter, completed-lens length and minimum focusing distance of the converted Funleader lens have not been published. These fields should therefore not be populated with the 56mm diameter, approximately 31mm length or 0.5m minimum focus specification of the original Contax G autofocus lens.

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Optical qualities

Rendering

The conversion retains the original Carl Zeiss Biogon T* 28mm f/2.8 optical block, so its fundamental imaging characteristics derive from the Contax G Biogon rather than from a newly calculated Funleader design.

The Biogon uses a strongly symmetrical wide-angle arrangement with the rear optical group positioned relatively close to the image plane. This construction contributes to the compact dimensions and strong geometric correction associated with the original lens.

Sharpness

Independent testing of the original Contax G28 optical generation has found strong central resolution from f/2.8, with good performance extending across the image as the lens is stopped down.

Because Funleader does not modify the optical assembly, the conversion housing should not be treated as introducing a separate optical performance generation. Practical results can nevertheless depend on the condition of the donor optics and accurate mechanical calibration of the conversion.

Contrast and color

The retained optical assembly uses Carl Zeiss T* multicoating. Independent evaluations of the original lens generally report strong contrast and saturated color reproduction.

Individual donor lenses are several decades old, so coating condition, haze and other age-related factors can affect individual converted examples.

Bokeh and transitions

The original Contax G28 uses a six-blade diaphragm. At smaller apertures, the geometry of the six-blade opening becomes increasingly visible in out-of-focus highlights.

As a 28mm f/2.8 wide-angle lens, strong background isolation is not a primary characteristic of the design.

Flare resistance

Independent testing of the original Biogon indicates reasonable resistance to flare for its generation, although direct bright light can produce flare or ghosting.

Condition and cleanliness of the original donor optical assembly can materially affect practical flare performance.

Distortion and vignetting

One of the notable characteristics of the Contax G Biogon design is its strong geometric distortion correction. Independent tests generally find very low visible distortion.

Vignetting remains visible at wider apertures and decreases as the aperture is stopped down.

Digital use

The original Contax G Biogon was designed for film. Its symmetrical optical construction places the rear elements close to the image plane, causing light to reach the edges of some digital sensors at comparatively steep angles.

On some full-frame digital cameras this can produce peripheral color shift, often visible as cyan, blue or magenta coloration toward the corners, together with increased edge shading. The severity depends strongly on the sensor stack and camera body.

This behaviour is a characteristic of the original Contax G optical design and is not created by the Funleader conversion.

The Leica M conversion nevertheless makes the optical block substantially easier to operate on compatible rangefinder cameras by adding a conventional manual focusing mechanism and mechanical rangefinder coupling.

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History

Development and Launch

The Carl Zeiss Biogon T* 28mm f/2.8 was introduced for the Contax G autofocus rangefinder system during the 1990s. The lens combined a compact symmetrical wide-angle design with autofocus operation controlled by the Contax camera body.

Unlike conventional Leica M lenses, Contax G lenses lack a normal manual focusing helicoid. Effective conversion to Leica M therefore requires more than a simple mount adapter.

Funleader and Mr Ding Studio began addressing this problem with a non-destructive conversion for the Contax G35 in 2019. The system transferred the original optical block into a dedicated manual focusing housing while maintaining the possibility of returning the optics to their original Contax configuration.

A G45 conversion followed in 2021. Development continued with subsequent mechanical revisions to the G35 system.

In March 2026, Funleader and Mr Ding introduced the corresponding conversion system for the Contax G Biogon 28mm f/2.8. This completed a three-lens conversion series covering the G28, G35 and G45.

Production Evolution

The G28 conversion incorporates lessons from the earlier conversion systems but was designed specifically for the mechanical requirements of the Biogon 28mm optical assembly.

Unlike the heavier brass construction used in some earlier Contax conversion housings, the G28 combines an aluminum exterior with a brass internal focusing structure. This reduces overall weight while retaining brass in the components responsible for focusing operation and mechanical stability.

The documented conversion housing weighs approximately 113g without optics. When fitted with the original G28 optical assembly, the complete converted lens weighs approximately 168g.

The original Carl Zeiss optical construction remains unchanged.

Special Editions/Variants

The documented Funleader production version uses a black exterior finish.

Funleader currently markets the G28 primarily as a conversion kit, requiring the owner to supply an original Contax G28 optical assembly. A completed conversion using the specified optical block represents the same commercial conversion identity and does not require a separate LeicaLensList profile.

Mr Ding Studio also markets the jointly developed G28 conversion system directly. Because the products are separately marketed through different commercial partners, the Mr Ding version may be maintained as its own LeicaLensList commercial conversion profile while documenting the shared development history.

No separate Funleader optical generation or officially designated V2.0 version of the G28 conversion has been documented.

Collector Notes

The converted lens should not inherit the physical specifications of the original Contax G autofocus barrel.

The original Carl Zeiss Biogon T* 28mm f/2.8 weighs approximately 150g, measures about 56mm in diameter and approximately 31mm in length, and focuses to 0.5m. These specifications describe the original Contax G housing and must not automatically be assigned to the Funleader Leica M conversion.

For the converted configuration, approximately 168g is documented with the original optical block installed. Overall diameter, length and minimum focusing distance remain insufficiently documented and should therefore remain Unknown until reliable conversion-specific specifications become available.

The G28 should also not be confused with the later Zeiss Biogon T* 28mm f/2.8 ZM. The ZM is a separate native Leica M optical design using eight elements in six groups, while the converted Contax G lens retains the earlier seven-element, five-group Contax optical formula.

The Funleader conversion is likewise distinct from other commercial rehousings of the Contax G28, such as the Omnar CG28-28. Although these products may use the same donor optical design, their focusing housings, mechanical construction, commercial provenance and collector identities differ.

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Sources

• Funleader, Contax G28 Leica M Conversion Kit (Non-Destructive)myfunleader.com →
• Mr. Ding Studio, Contax Biogon G28/2.8 for Leica M-Mount Conversion Kitmrdingstudio.com →
• Carl Zeiss, Biogon T* 2.8/28 Contax G Datasheetzeiss.com →
• PetaPixel, You Can Convert Your Contax G 28mm f/2.8 Zeiss Lens to M-Mountpetapixel.com →
• Digital Camera World, Funleader Announces Major Milestone With Contax G to Leica M Camera Conversion Kitdigitalcameraworld.com →
• Phillip Reeve, Carl Zeiss 28mm f/2.8 Biogon T* Reviewphillipreeve.net →
• Casual Photophile, Carl Zeiss G Mount 28mm f/2.8 Biogon Reviewcasualphotophile.com →