Here's an interesting one that came out of the recent X system summit by Fujifilm.

They came up with something that is called the “value angle” when talking about the various lens mounts out there for photography.

What is the value angle?

The formula for value angle = 2 × arctan ((mount diameter – sensor diagonal) / (2 × flange distance))

Which in layman's terms, is a calculation that takes into account the flange distance of a mount. the size of the mount itself against the size of the image sensor.

In Fujifilm's mind, this formula tells us and them how difficult it is to design top performing lenses for a particular mount. The winner? The EF-M mount from Canon. The latest RF mount from Canon didn't do as well according to the formula.

 Company Mount Value Angle Format Flange Distance Diameter Sensor Diagonal Canon EF-M 58.6 APS-C 18 47 26.8 Leica L (TL) 54.5 APS-C 20 48.8 28.2 Sony E (NEX, E-mount) 52.9 APS-C 18 46.1 28.2 Fujifilm X 48.1 APS-C 17.7 44 28.2 m43's (Olympus, Panasonic,etc) MFT 46.0 Micro 43's 19.3 38 21.6 Nikon Z 40.2 Full Frame 16 55 43.3 Canon RF 30.0 Full Frame 20 54 43.3 Fujifilm G 21.6 44×33 26.7 65 54.8 Hasselblad XCD 17.6 44×33 20 61 54.8 L-mount Alliance L 15.7 Full Frame 20 48.8 43.3 Sony E (FE) 8.9 Full Frame 18 46.1 43.3

As you can see in the table above, the EF-M mount takes the top prize, while the RF mount comes in second for flexibility behind Nikon's Z mount and well ahead of Sony's E(FE) mount.

Canon News shows us what the EF-M mount would look like using this formula for the EF-M mount with a full frame image sensor.

 Company Mount Value Angle Format Flange Distance Diameter Sensor Diagonal Canon EF-M 58.6 APS-C 18 47 26.8 Canon EF-M 11.7 Full Frame 18 47 43.3

I think this likely shows us that the EF-M mount was never intended to be full frame and makes the introduction of the RF mount much needed for the full frame mirrorless application.