The Canon IIb is a 35mm rangefinder camera produced by Canon and introduced in April of 1949. It uses the M39 threaded lensmount, the same mount as early Leica rangefinder cameras.
The IIb was the successor to Canon's previous SII model, and for a short time this camera was even referred to as the SIIb before it became known simply as the IIb. It uses a combined rangefinder and viewfinder, something Leica wouldn't accomplish until 1954 with the introduction of their M3 camera. The IIb also added an additional feature of variable magnification in its viewfinder. Via a switch under the rewind knob, the viewfinder could be set to "F" (.67x magnification), 1x or 1.5x. The higher magnification settings would be used for more critical focusing but could also be employed for composition with telephoto lenses (the 1x settings worked for 100mm lenses while 1.5x was appropriate for 135mm lenses).
Other features of the IIb included dual shutter speed dials with high speeds ranging from 1/30th to 1/500 (and Bulb) being located on the top dial with a slow speed dial on the front face of the camera. The slow speed dial had shutter speeds of 1/20th to 1 second. Like Leica cameras of the time, the IIb was a base-loading camera. The user removes the baseplate, loading film in through the bottom of the camera. A removable take-up spool is pulled out of the camera, film is attached to it and the spool and film can are both pushed back up into the camera.
The top of the IIb has an accessory shoe for holding auxiliary viewfinders, such as the one seen in the above image. It could also hold flashes and sync them with the shutter. The camera does not have a PC port for off-shoe synchronization.
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Canon IIb with Serenar 50mm f/1.9
When discussing rangefinder cameras that use the M39 threadmount, most folks divide the cameras into one of two categories: Leicas and "Leica copies." This is understandable as Leica was the king of the 35mm rangefinder market and is still the most common camera that comes to mind when discussing this type of film cameras. That latter category of "Leica copies" or "Like a Leica, but not a Leica" is a fair categorization of many of the cameras that fall under this umbrella. At the same time, it vastly undersells and under-appreciates many of the cameras that might get labeled as such. A good example of this is the Canon IIB.
The Canon IIB was the successor to the Canon S II. In fact, it was initially even called the Canon S IIB before it was renamed simply as the IIB. Introduced in April of 1949, this Canon had a relatively significant feature that surpassed its Leica competition - its rangefinder/viewfinder. Until the introduction of the Leica M3 in 1954, Leica cameras had different windows for rangefinders and viewfinders. Additionally, those viewfinders could only accommodate a single 50mm focal length, in terms of composition.
Canon, meanwhile, had been combining the rangefinder and viewfinder into one window since 1946, with the aforementioned Canon S II. The IIB continued that innovation and brought another feature into the game: variable magnification viewfinders. The IIB has a switch near the rewind knob that allows the user to change viewfinder magnification between .67x, 1x, and 1.5x settings. This magnification allowed for more ease when composing with telephoto lenses or for magnified, precise focusing. It would take Leica another five years to get a similar feature in their cameras.