For inspiration: http://killzonekid.com/arma-scripting-tutorials-get-zoom/

You can calculate current FOV using screenToWorld to measure the distances between two points on the edges of the screen, and those points and the camera. FOV can then be calculated using the law of cosines. The catch is it only works if the edge points are on the ground (same as Killzone Kid's method)

Zoom/magnification can be derived from the current and baseline FOV, but as with KK's method is does require a conversion factor unless the screen aspect ratio is 16:9.

I used these methods to compare FOV/zoom in #9173