The sensor's vertical extent, millimeters.
Fixed, and the reason it is a constant rather than a per-lens dial is that
every angle in the repository was converted through it once: change it and
lensForFov returns different focal lengths for the same shipped
compositions, which is a change of representation nobody asked for. 24 mm is
the height of a full-frame sensor, so the focal lengths this produces are the
ones a photographer would recognize — the flight lens comes out at 18.84 mm,
which is a wide prime, and the cinematic one at 28.97 mm, which is a normal.