Archived posting to the Leica Users Group, 1996/04/13

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Subject: M lens for 645 format
From: (TK Chan)
Date: Sun, 14 Apr 1996 02:11:21 +0800 (HKT)

Many have asked for more information on the conversion. The following is
what I understand:

For any 135 format lens suitable for conversion, it must satisfy 2
requirments - 

1) the focusing distance between the rear lens piece and the image on the
film surface must be longer than the 645 or 6X6 camera body's mirror
housing; and

2) the entire unit of lens must be detachable intact with its aperture

Because of these restrictions, not all Leica lenses are usable for this
purpose. The R lenses are definitely out of question. M lenses from 90mm
upwards designed for visoflex use which have detachable lens unit are most
suitable for adapting to use in the Mamiya 645. It is said that the Elmar
M65mm f/3.5 can also be fitted but I have yet to get one.  The current
models of Tele-Elmarit 90 f/2.8, Summicron 90 f/2, Tele-Elmar 135 f/4 and
the Elmarit 135 f/2.8 are unfortunately not suitable because although their
lens units are detachable, the rear lens piece is too close to the image plane.

The idea of the conversion is to put the lens unit in a new focusing tube
which is suitably adjusted to the correct length and having a Mamiya rear
mount for attaching it to the camera body. If anytime you wish to use the
lens on the M body again, simply re-fit the lens unit to its original
focusing tube.

There is no ready made conversion tube available and I have it made by my
friend who is an expert in converting these equipment.  I was given to know
that the screw threads of each of those old time Leica lenses are not of
exact measurement so the conversion tube has to be made to exact match with
the lens unit's screw threads.

My Elmar 135 is fitted to a focusing tube converted from an old Tamron 135mm
lens. A screw mount is fitted to the front of the tube for attaching the
lens unit and a bayonet mount at the rear is made for fitting to the Mamiya
body. The focusing distance of the whole lens must be exactly adjusted to
infinitive. With is lens, I can still use the automatic exposure of the
camera (fitted
with an AE finder) but have to suffer the slight inconvenience of stopping
down to the desired aperture for the exposure measurement.

I spent about $6000(Hong Kong Dollar) for the lens and the conversion tube.
I consider it
extremely worthy of the money spent because the original Mamiya A150mm f/2.8
costs up to HK$9000 and the quality - you can imagine which is the winner.

I'll talk about the conversion of the 200mm later.

TK Chan