The camera’s field of view defines how much it sees and is a function of the focal length of the lens and the size (often called the format) of the digital sensor.
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.
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