CONTAX BIOGON 2.8/21 T*(GtoM)

This is a record of using a Kyocera CONTAX G mount 21mm f/2.8 lens converted to Leica M mount by MS-Optics (Miyazaki Kogaku) with a Leica M digital camera.

Gallery
Impression
Specification
Third party lens hood
Reference links

Gallery
CONTAX G BIOGON 21mm F2.8 Photo example

Impression
As shown in the upper specifications, the lens configuration is a symmetrical Biogon type, and the rear lens element is close to the sensor (film) surface, causing color casts on the periphery of some digital camera sensors (color casts can also be seen in the sample images).
The following three measures can be taken to deal with color cast, but it depends on how the camera is used.

Enjoy the results in monochrome mode.
Boldly drop the peripheral areas in case of color
Choose a color subject where the cast is not noticeable (give up on blue skies).

However, the retrofocus Elmarit 21mm, even at the same focal length, does not produce this kind of color cast, making it a difficult lens to use on Leica digital as well as the Super Angulon.
In this respect, when used with the EPSON R-D1 and LEICA M8, which have smaller sensor sizes, the lens becomes easy to handle, although the angle of view is about 32 mm because the periphery is eliminated.
This lens has excellent backlight resistance, probably due to its good coating, and even in situations where sunlight enters the subject, we rarely encounter flare or stray light.

Since the lens has been converted to Leica M mount, it can be used with various mirrorless cameras with the use of a mount adapter.
I have not seen any M-mount modifications for this lens except those done by Ms-optics.

Third party lens hood
A non-original rectangular lens hood purchased when I was using the original Biogon 21mm for G.
It has a screw-in stopper and can be attached to any 57mm diameter lens. The original Biogon for G had a 57mm lens tip, so it is thought that this hood was designed to fit this lens.
Since the front part of the lens barrel of the original Biogon for G does not rotate when the lens is focused (inner focus), a rectangular hood like this hood can be used.
Also, the outer diameter of the genuine CONTAX 55mm lens filter is 57mm to match the Biogon 21mm, and if the lens has a 55mm filter thread, this hood can be attached via this 55mm filter.
However, when the focus ring of the modified BIOGON 21mm is turned, the front part of the lens rotates, and this rectangular hood also rotates with it. Since it is not practical to change the position of the lens hood after fixing the focus, this hood is not used. It is a pity, because I like the appearance of the hood very much and would like to use it.

Specification
AF lens for CONTAX G converted to Leica M mount (distance meter linked) at MS-Optics.
With the modification, the AF function is eliminated and the lens becomes a manual lens. Also, the lens before the modification has an inner focus, but after the modification, the barrel head rotates to change focus.
It uses a common helicoid called MS-21.
There is no click stop on the aperture, which is a specification of MS-Optics lenses.
The aperture ring is located at the valley where the lens barrel nips in, so it is nice that the aperture value does not change inadvertently.
Another lens, the Planar 45mm modified by Ms-optics, has an aperture ring at the front of the lens, which can cause problems with unintentional changes in the aperture location.

Item	Value	note
focal length(mm)	21
Maximum aperture	2.8
Minimum aperture	16
Lens configuration	7groups 9elements
Minimum distance(m)	0.5	Works with the camera’s rangefinder from 0.8m to infinity.
Lens length(mm)	36
Lens max diameter(mm)	56
Filter diameter(mm)	55
Weight(g)	151