What is a Leica Thread?
In 1931, Leitz introduced a thread-mount of 39mm x 26tpi (threads per inch) for its Leica cameras. This combination of a metric diameter with an English imperial pitch was designed in order to forestall competition from aftermarket lenses being used with the Leica screw-mount rangefinder cameras. Although it was patented in the same year, other companies pirated the design and introduced lenses with the new 39mm thread. Presumably in their haste to introduce their own products with the Leica thread, some rival companies did not take the time to verify that the Leica thread’s pitch was indeed 26 threads per inch. They came out with products that used a metric pitch instead. These “almost” Leica thread lenses would bind to true Leica mounts (if forced.) For this reason, Leica thread lenses (not made by Leica) should be GENTLY screwed into their mounts. If resistance is felt, they should not be forced. At the same time, Leitz also marketed enlargers with the Leica thread in order to allow
