The Evolution of the Canon Mount
Canon Lens Mount versus the Leica Lens Mount:
Until 1951, the lens mount used by Canon on their rangefinder
cameras was not exactly the same as the Leica Thread Mount, often abbreviated as "LTM", and also referred to as the "universal"
mount, as the "M39 mount" and sometimes as the "L mount".
The Canon mount, referred to as the "J mount" had a diameter
of 39mm, identical to Leica. However, whether intentionally, or by miscalculation, the thread pitch of the J mount was
The Canon official history states:
"...The thread mount of 24 screw thread per inch with 1.058mm pitch was used instead of Leica specification."
The Leica thread mount, or "universal" mount was 39mm in diameter and had a thread pitch of 26 threads per inch.
(Thread pitch is the distance in millimetres between one thread and another on a threaded screw.)
Why the difference between Leica and Canon lens mounts?
It is said that Leica adopted their thread pitch based
on microscope thread standards set in threads per inch. It is said the the (British) Royal Microscopical Society had
already propogated standards for various sizes of microscope objective lenses, which are screwed into a microscope barrel. This
was a number of years before the 1947 ISO standardization of many screw threads in diverse technical areas, which are used
Canon may have believed that the Leica mount was a 25.4
pitch (i.e. 1.058mm x 24mm = 25.4) this being the number of millimeters in 1 inch. This explanation would account for
the use of such a precise 1.058mm pitch, which is otherwise not an obvious choice.
Use of the J mount by Canon
In any case, J mount was the mount into which the
Nikkor focusing mount screwed into the Canon Hansa body, with the Nikkor lenses bayonetting into the focusing mount.
This Nikkor mount was further used in the Canon Model S and the Model NS.
Later Canon models which did not use the Nikkor focusing
mount still used the J mount. Thread mount lenses screwed directly into the J mount on the Canon Model J,
Model J-II and the Model S-II bodies.
Change of Canon to be compatible with Leica screw mount.
The J mount and Leica mount continued to be incompatible
until about 1947 with the introduction of the "semi-universal" thread mount on later Canon S-II examples. The thread
pitch of the "semi-universal" mount was closer to 26 tpi, with thread construction close enought to the Leica "universal"
mount to be interchangable. Canon went to fully standard 39mm x 26 tpi mount with models produced in late
1951. This mount incompatiblity was only as to the threading of the mount, since the lens to film plane distances were
identical to Leica cameras.