What is mFT FoV?

Measured to the best of my ability with a 20mm lens (because it has fairly strong distortion correction):

Distance from the sensor plane to the horizontal measuring tape: 20 in.

horizontal field of view: 19 7/32 in - 3 7/32 in = 16 7/32 in = 16.21875 in.

diagonal field of view: 16.21875 / 4 * 5 = 20.2734375 in.

Diagonal angle of view: 2 * arctan (20.2734375 / 2 / 20) = 53.7550599 degrees

Specified: 57 degrees

How significant is this deviation?

Working back from 57 degrees, the horizontal field of view at 20 in should be:

tan(57/2º)*20*2/5*4=17.374582388429978 in.

The deviation is

(17.374582388429978-16.21875)/17.374582388429978*100 = 6.7%

It should be taken into account that the 20mm lens exhibit observable "focus breathing," and the angle of view at infinity is somewhat larger. How significant it is is up to the user. I was not planning to use the camera for triangulation anyway.