In Fig. 1, the formula for the $\alpha$, angular field of view is \[ \alpha = 2\arctan\frac{d}{2f} \] in which $d$ is the diagonal of the square format photograph and $f$ is the focal length of the camera.

In Fig. 1, the formula for the $\alpha$, angular field of view is \[ \alpha = 2\arctan\frac{d}{2f} \] in which $d$ is the diagonal of the square format photograph and $f$ is the focal length of the camera.