Omnar CG21-28 (Contax G 21mm conversion)

The Omnar CG21-28 (Contax G 21mm conversion) is a M-mount lens for Leica rangefinder cameras. Leica price index ↗

Reference maintained by Thomas Boots

Model number(s): CG21-28

Focal Length: 21mm

Aperture: 𝑓/2.8

Release Year (from): 2024

Production Year (to): 2025

Diameter: 57 mm

Length: 36.5 mm

Minimum Focus Distance: 0.2m

Elements in Groups: 9/7

Aperture Blades: 7

Mount: M

Material Weight: Brass, 300g

https://omnarlenses.com/

Omnar CG21-28 (Contax G 21mm conversion)

The CG21-28 is not a new optical design but a rehousing project: Omnar Lenses takes the optical block of the Carl Zeiss Contax G 21mm f/2.8 Biogon, a lens that could only autofocus on the Contax G bodies it was built for, and rebuilds it into a manual-focus Leica M-mount barrel. Omnar is a UK venture run jointly by Hamish Gill of the 35mmc website and Chris Andreyo of Skyllaney Opto-Mechanics, and the conversions reuse the existing exterior metal frame and original Zeiss glass of the donor lens combined with machined brass components made in the UK [1][2]. The Biogon design behind it is widely regarded among rangefinder users as one of the strongest ultra-wide lenses of its era, with reviewers and long-term owners praising its colour, resolution and distortion control [3].

Mechanically, the conversion is built around a custom all-brass helicoid. This gives rangefinder-coupled focus from 0.65 m to infinity, and adds an uncoupled close-focus capability down to 0.2 m intended to be used with live view or an electronic viewfinder, effectively allowing the 21mm and 28mm Biogons to be used at near-macro distances they could never reach on the original Contax G mount [1]. The lens carries seven aperture blades and uses a 55 mm filter thread, and it remains a fully manual lens once converted: the original autofocus screw drive of the Contax G system is not retained, so focus is set by hand using the rangefinder or an external finder [1][3]. The CG21-28 is offered in black or silver finishes [1].

Because the Contax G 21mm and 28mm Biogons share a rear element that is comparatively large, they could not be handled by Skyllaney's standard Omnar workflow and required purpose-designed aperture control assemblies before conversion was possible; the company flagged these two Biogons as a later, more complex project while it developed the necessary parts [2]. As a small-batch, made-to-order rehousing rather than a mass-produced lens, individual examples are built from a customer's own donor optic or from donor lenses sourced by Omnar, so condition and provenance of the underlying Zeiss block matter as much as the new barrel.

Optical qualities

Rendering The optical performance of the CG21-28 derives from the original Contax G Biogon rather than from any new Omnar glass. That lens is consistently described by experienced users as having strong colour depth, high resolution and well-controlled distortion, with a sense of dimensionality even at full aperture [3]. Omnar itself regards this generation of Zeiss lenses as among the best ever made [1]. Reviews of the underlying design have historically been mixed in one respect: opinions differed over how truly symmetrical its Biogon layout is, and on mirrorless cameras the original optic is known to be demanding in the far field, where edges and corners can require stopping down and careful focusing, partly because the rear element sits close to the sensor [3][4].

Digital use On digital M bodies and adapted mirrorless cameras, the close rear-element position of the Biogon has been associated with weaker corner sharpness and colour shift toward the frame edges at infinity, a trait reported with the unconverted Contax G 21mm; near and mid distances are less affected [4]. The Omnar conversion does not alter the optics, so these characteristics carry over, while the added 0.2 m close-focus ability is specifically described as a live-view or EVF feature [1].

History

Development and Launch Omnar Lenses grew out of a collaboration between 35mmc and Skyllaney Opto-Mechanics that was later incorporated as a separate business, with the website's founder becoming a half owner [5]. Skyllaney's Omnar platform was introduced as a way to convert non-rangefinder optical blocks to M bayonet with full rangefinder coupling, and the company publicly identified the Contax G 21mm and 28mm Biogons as conversions it was working toward once suitable aperture assemblies had been designed [2]. The CG21-28 reached the market in this later wave, with the product listing and sample images dated to 2025 [1].

Special editions No major factory special variants are documented for this conversion beyond the choice of black or silver finish offered at the time of order [1].

Collector Notes Prospective buyers should treat the CG21-28 as a hand-built rehousing of a vintage Zeiss optic, which means the value rests on both the integrity of the new brass barrel and the condition of the donor Biogon glass and coatings. The lens focuses manually and couples to the rangefinder only from 0.65 m to infinity, with the sub-0.65 m range down to 0.2 m being uncoupled and reliant on live view or an EVF, a point worth understanding before purchase [1]. Because Omnar conversions are produced in small numbers on long lead times, originality questions center on which donor lens was used and how it was finished rather than on serial-number ranges [2]. One discrepancy is worth noting for the record: Omnar's own product page cites a rangefinder-coupled minimum focus of 0.65 m, whereas LeicaLensList records the lens's minimum focus distance as 0.2 m, the uncoupled close-focus figure [1].