Canon's Serenar rangefinder tele — 135mm f/4 in Leica Thread Mount, manual focus.
The Canon Serenar 135mm f/4 is a telephoto rangefinder lens made in Japan for the 39mm Leica screw thread. It dates from the late-1940s Serenar period of Canon's rangefinder line, among the company's early long lenses. It gave Leica-thread and Canon rangefinder users the longest reach in the early screw-mount range, typically framed with an accessory finder.
This is a manual-focus, rangefinder-coupled Leica Thread Mount lens with a 135mm focal length and a maximum aperture of f/4. The mount is the 39mm Leica screw thread (LTM / L39 / M39). Focus and aperture are set on the barrel by hand. Element count, weight and filter thread are omitted here because they are not verified from the gap data.
At 135mm this is a proper telephoto for rangefinder use, good for distant subjects, tighter portraits and strong perspective compression. The f/4 aperture keeps it manageable and points it toward good light, where slower teles of this class tend to be sharp when stopped down slightly. Rangefinder focusing at 135mm rewards care, as depth of field is thin at distance.
Buying used, inspect these early Serenar teles for haze, fungus and balsam separation in the cemented groups. Check the coatings for cleaning marks and wear, look for oily aperture blades, and confirm the focus helicoid is smooth. Rangefinder-coupling accuracy is most demanding at 135mm, so test it carefully on a body. It adapts well to Leica M via M39-to-M and to mirrorless with M39-to-E, M39-to-Z or M39-to-RF adapters.