Horizontal and Vertical FOV Calculator
As is commonly done with cameras, DJI advertises the Field of View (FOV) of their lenses using the diagonal angle. This FOV value corresponds to an aspect ratio of 4x3 (and sometimes 3x2) which is typically used for photos.
It is often desirable to know either the horizontal FOV or vertical FOV or both. To compute either the horizontal or vertical FOV, it is tempting to use the Pythagorean Theorem to compute the ratio of aspect-horizontal to aspect-diagonal and then multiply the diagonal FOV by that ratio to obtain the horizontal FOV but that would be incorrect.
The reason the above method would be incorrect is because the FOV angle, when projected onto a flat plane has a non-linear relationship with distances along that plane. For example, a one-degree change of FOV when the FOV is small results in a relatively small linear change on that flat plane. However, a one-degree change of FOV when the FOV is large (closer to 180) yields a massive change on that same flat plane.
The tangent of the half-FOV angle would be equal to half the aspect over the adjacent side. Applying this rule to both the diagonal and the horizontal yields two equations that can be combined to form one equation relating the FOV angles to the aspect ratios. The details of this method can be seen here.
This web application will perform these calculations for any of the DJI models.
Details