How far the framing pose backs off at fov relative to the 50° pose: the larger of
tan(25°)/tan(fov/2): the same visible half-height at the target plane (what a dolly keeps), and
sin(25°)/sin(fov/2): the same bounding-sphere margin.
Narrower than 50° the first wins; wider, the tan rule alone would put the camera inside the sphere
(at 120° it lands at 0.72·r), so the sphere rule takes over. Exactly 1 at 50.
How far the framing pose backs off at
fovrelative to the 50° pose: the larger of