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Canon's EOS-M mount is the best designed mount, according to Fujifilm | Canon News
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Canon's EOS-M mount is the best designed mount, according to Fujifilm
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Canon's EOS-M mount is the best designed mount, according to Fujifilm

In a presentation at the X system summit, Fujifilm came up with Value Angle.

In essence, the value angle takes the mount's flange distance and the size of the mount against the size of the sensor.  All three of those parameters define how easy or hard it is to design lenses.

The formula of;

Value Angle = 2 × arctan ((mount diameter - sensor diagonal) / (2 × flange distance))

is used to calculate out the Value Angle of a particular mount.

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 44x33 26.7 65 54.8
Hasselblad XCD 17.6 44x33 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

The concept around this is that the shorter the light has to travel between the lens and the sensor to the corners, the easier it is to design the lens to be free of aberrations.

The Canon EOS-M mount because it's APS-C and also Canon's APS-C sensors are slightly smaller than other manufacturer's APS-C sensor ends up having the best score.  It should be noted that these values are only approximate because in actuality they should use the inner diameter of the lens mount, but that value is largely unknown across the mounts.  So for approximation, the outer diameter is used for the above chart.

Interestingly using the formula we see why Canon struggled to use the EOS-M mount as a full frame mount.  If they did, their Value angle would have been:

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

Instead, the value angle of the Canon RF mount is 30.0, and the second most flexible full frame mount behind Nikon's Z mount.

This doesn't mean that for instance, the Sony E mount can't have specific lenses for its mount, or that it's inferior, it's just that the design challenges are more as you decrease the size of the value angle.  So while it's obvious that Sony has developed some amazing lenses in the G-Master series, it just may be an easier process for Canon to catch up and also design excellent lenses for both the EOS-M and EOS-RF mounts.  Each of Canon's mirrorless mounts is specifically tuned for the sensor size in which the mount uses.

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