Attaching EF lenses to M mount camera requires an adapter – which adds a bit to the bulk of a small camera, but is also an interesting opportunity, since it is possible to fit new electronic or optical functionalities inside that middle piece.
I have both the official, Canon-made “EF-EOS M” mount adapter, which keeps the optical characteristics of the lens similar to what they would be if used on an EF-S mount camera (crop and all). The other adapter is “Viltrox EF-EOS M2 Lens Adapter 0.71x Speed Booster” (a real mouthful), which has the interesting capability of multiplying the focal length by factor of 0.71. This is a sort of “inverted teleconverter” as it reduces the image size that the lens produces, allowing more light to fit into the smaller (APS C) sensor, and almost eliminates the crop factor.
Most interestingly, as the booster collects more light into the sensor, this also has an effect of increasing the maximum aperture of my EF/EF-S lenses in an M mount camera. When I attach Viltrox into my 70-200 mm F4, it appears to my M50 camera as an F2.8 lens (with that constant aperture over the entire zoom range). The image quality that these “active speed booster adapters” produce is apparently a somewhat contested topic among camera enthusiasts. In my personal, initial tests, I have been pretty happy: the sharpness and corner vignetting also appear to be well controlled and the images produced of rather good quality – or good enough for me, at least.
When I put this into my 50 mm F2.8 portrait lens, this lens functions as having F1.2 maximum aperture. This is pretty cool, e.g. the capability to shoot in lower-light conditions is much better this way, and the narrow depth of field is similar to much more heavy and expensive, full frame camera system when using this adapter.
In my tests so far, all my Canon EF lenses have worked perfectly with Viltrox. However, when testing with the Tamron 16-300 mm F/3.5-6.3 Di II VC PZD super-zoom lens, there are issues. The adapter focuses light in a wrong manner when using this lens, and the result is that the corners are cut away from images (see the picture below). So, your mileage may vary. I have written to Viltrox customer service and asked what they suggest in the Tamron case (I have updated the adapter into the most recent available firmware – this can be done very simply using a PC and the built-in micro-usb connector in the adapter).
You can read a bit more about this technology (in connection to the first, Metabones product) from here: https://www.newsshooter.com/2013/01/14/metabones-speed-booster-adapter-gives-lenses-an-extra-fstop-and-nearly-full-frame-focal-lengths-on-aps-c-sensors/