Soothsayer

vector campos = chv('campos');
vector objectpos = chv('objectpos');

// Initial horizontal aperture
float H_initial = 20.955;

// Object size (A) and distance (R) from the camera
float A = chf("object_size"); // Example channel referencing object size
float R = distance(campos, objectpos);
// Field of View (FOV) needed to fit the object at distance R
float FOV_needed = 2 * atan(A / (2 * R));
// focal length to achieve the required FOV with the initial horizontal aperture
@focal_length = H_initial / (2 * tan(FOV_needed / 2));