M modified CONTAX G Biogon T* 28mm

A review and sample photos of the CONTAX G Biogon 28mm F2.8 lens, modified by Avenon Optical to an L39 mount, used with digital and film cameras.

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Table of contents

Gallery
Review
Specification and more
Reference links

アマゾン・アフィリエイト・リンク
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Gallery

The following cameras and films were used to take the example photographs.

HEXAR RF LIMITED +FUJIFILM FUJICOLOR 200
Leica M-P typ 240
SONY α7SII

1．Overview

The CONTAX G Biogon 28mm F2.8 is an autofocus lens released by Kyocera along with the Contax G1 in 1994. Here, we introduce a lens that was modified by AVENON Optics to Leica L mount (linked to rangefinder camera rangefinder).

AVENON, which modified this lens, was founded around 1982 and is a company that no longer exists as of 2024.

AVENON has previously released two original lenses for the Leica L39 screw mount, the AVENON-SUPER-WIDE 21mm and the AVENON 28mm, as well as a Leica M mount pinhole lens, which are still traded on the second-hand market.

AVENON offered a service to convert CONTAX G mount 28mm lenses to M mount and modify them to link with Leica M type camera rangefinders. This lens is one of those modified lenses.

The feature of the Avenon modified lens is that it uses the original lens barrel almost as it is, while adding a Leica rangefinder linkage mechanism to the G lens. Therefore, the aperture ring is the same as the original and stops with a satisfying click every click stop. The aperture blades also follow the original six-blade design.

2．Usability

The CONTAX G Biogon 28mm F2.8 M-mount modified is modified to work with the rangefinder of a rangefinder camera, allowing you to focus using the double image in the viewfinder.

The modified lens’s minimum focusing distance has been changed from 0.5m (the original lens) to 0.8m to match the rangefinder coupling range of rangefinder cameras.
This is thought to be because the lens was modified during the heyday of film cameras, before mirrorless cameras, and there was no point in extending the lens to a point where it was not coupled to the rangefinder of Leica M cameras.
Although the value of 0.8m falls short of the 0.7m rangefinder coupling limit of Leica M cameras, it is thought that 0.8m was chosen due to the fact that it is a wide-angle 28mm lens, and concerns about discrepancies between the viewfinder and the actual image when taking close-up shots due to parallax, as well as a lack of focusing accuracy when shooting close-ups.

When using with a mirrorless camera, the 0.8m minimum focusing distance can be overcome by using an auxiliary helicoid.

Before I got the black version I currently own, I owned the champagne gold version.

The sad thing about the black Biogon 28mm lens, and this is purely cosmetic, is that the silver aperture range ring doesn’t match the black body. If this part were a black plate with white lettering, the design would be more complete. The modified helicoid for manual focus is very smooth and has just the right amount of weight, making focusing completely effortless.

Image quality is the same as the original G-BIOGON 28mm, but the rear element extends into the film chamber, a shape commonly seen in wide-angle lenses for rangefinder cameras.

The lens construction is nearly symmetrical, with three elements in the front group and four in the rear group. This configuration minimizes subject distortion during shooting, allowing for comfortable capture of grid-like objects.

However, this lens was not designed with the digital age in mind, and weaknesses include color casts on the periphery of the image and purple fringing around the subject, depending on the camera’s sensor.

Ms-optics, which has modified other Contax G-mount lenses, also modifies the G Biogon 28mm. However, since they obtained the modified Avenon lens first, they did not request a modification for the G Biogon 28mm.

3．Summary

Sum up the CONTAX G Biogon 28mm F2.8 M-mount conversion, the biggest advantage is that the focus position can be determined by the overlapping of the double image of a rangefinder camera. Another advantage is that the lens’ versatility is improved because there are more types of mount adapters for the M-mount than for the G-mount.

Specifications, considerations, etc.

In 2005, CARL Zeiss released the BIOGON ZM 28mm manufactured by Cosina. The lens length from the mount surface is 38mm, while the modified G Biogon 28mm has a lens length from the M mount surface of 31.7mm, making the modified G Biogon more compact.

The lens length changes depending on the flange back, so the flange back of the G mount is 29mm, and the lens length from the mount surface of the original G Biogon is 30.5mm, while the flange back of the Leica M mount is 27.8mm, which is 1.2mm longer, so it is 30.5mm + 1.2mm = 31.7mm.

The reason why the modified G Biogon is compact is because the front lens of the Biogon ZM is larger and the rear lens is slightly forward, as can be seen from the figure below (the image size has been adjusted to match the actual dimensions) created from the PDF officially distributed by Zeiss. This design is thought to aim to ensure sufficient peripheral light and reduce vignetting, and to improve telecentricity, which allows light that passes through the lens to enter the image sensor directly, by moving the rear lens forward.

The Biogon 28mm ZM, with its slightly moved forward rear lens, is presumably less likely to cause color casts in the peripheral areas.

The lens construction diagrams are quoted from each company’s materials, and the sizes have been adjusted by us, so they are not exact.

Item	*CONTAX G Biogon 28mm F2.8*	*BIOGON 28mm F2.8 ZM*
focal length(mm)	28	28
Maximum aperture	2.8	2.8
Minimum aperture	22	22
Leaf blade	6	10
Lens configuration	7 elements in 5 groups	8 elements in 6 groups
Minimum distance(m)	0.5	0.5
Lens length(mm)	30.5	38
Lens max diameter(mm)	56	53
Filter type	46	46
Weight(g)	150	220
Hood	GG-1	Carl Zeiss Lens Shade 25mm/28mm
Lens mount	CONTAX-G	ZM
Release date	1994	2005.4.29
Price	¥57,000	¥95,000

Reference links

Zeiss Biogon description page on Wikipedia
Zeiss Photography Historical Products(There is a data sheet for the CONTAX G lenses in the middle of the linked page.)
FUJIFILM 200, G BIOGON 28mm M-mount modified for shooting・Shige’s hobby
BIOGON T* 28mm ZM vs BIOGON T* 28mm G・Shige’s hobby
AF Biogon T* 28 +CONTAX G1,G2・Shige’s hobby

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