There is a lot of chatter lately about Canon and Nikon tardiness in the mirrorless world. On one side, Canon seeked feedback from their user about what they want in a pro mirrorless camera. On the other side, Nikon is announcing a full frame mirrorless camera with a new lens mount dubbed “Z-mount”. In both sides there is one topic that seems to come back into the discussion: the camera should be directly compatible with the SLR mount (EF for Canon, F for Nikon). While it is clearly understandable why users would want that, let’s explain on why it is not a good idea, and why the mount adapter is the best compromise — compromise that Canon made for the EOS-M.
A lens mount is defined by a certain number of attributes ; flange distance is the one that matters here.
The flange distance or register distance is the distance between the lens mount ring on the camera body and the focal plane (the surface of the sensor or the film). It is a fixed dimension for the lens mount.
On an SLR camera, you have the mirror box between the lens and the sensor, defining a minimum flange distance, while a mirrorless doesn’t have the mirror box. This is why in general a mirrorless camera has a much shorter flange distance even with a similar sensor size.
For example, Sony E-Mount is 18mm while Sony A-Mount (former Minolta SLR) is 44.5mm, as Canon EF-M is 18mm while Canon EF and EF-S are 44mm. In both cases the flange on SLR mount is more than twice as large as on the mirrorless.
What would a DSLR mount do on a mirrorless system? It would increase the thickness of the body in way that the camera wouldn’t be that much more compact. The real example is Pentax mirrorless Pentax K-01. With a flange distance of 45.46mm for the K-Mount, the camera is bulkier than it should with a thickness (depth) of 59mm. In comparison, the Canon EOS-M100 is 35mm thick (deep) as it uses the EF-M mount, designed for mirrorless.
That’s why using a DSLR mount for a mirrorless system, meant to be more compact, isn’t a good design choice. Offering an adapter that support all the features of their DSLR mount is, on the other hand, the best compromise that a camera maker can do, almost equivalent to the practicality of using native mount, but the advantage of the size: the extra bulk only comes if you need it.