New 'RF' mount
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New 'RF' mount

The Canon EOS R debuts a brand new lens mount - Canon's fourth 'full-frame' mount, if you include the long since retired manual focus FL/D and R. The transition from FD to the totally new (and totally incompatible) electronic EF mount in the late 1980s was quite a jolt for some long-time Canon shooters, but EF to RF looks like it'll be considerably smoother.

Like the 30+ year-old EF mount, the RF mount is based around a throat diameter of 54mm but with a much shorter flange-back distance of 20mm (compared to 44mm). This is compared to 46mm and 18mm for EF-M, and the longer flange-back distance of the full-frame RF standard would appear to make Canon's two mirrorless mounts incompatible with (i.e., unadaptable to) one another.