Drivers

Drivers are a way to control values of properties by means of a function, or a mathematical expression.

As an example, the rotation of Object 1 can be controlled by the scale of Object 2. It is then said that the scale of Object 2 drives the rotation of Object 1.

Not only can drivers directly set the value of a property to the value of a different one, they can also combine multiple values using a fixed function or a Python expression and further modulate it with a manually defined curve and/or a modifier stack.

You can read more about driver in the offical documentation page.

Using drivers

First, we need an object of a type which can be imported into Godot with the glTF importer.

Create an object, such as a default Cube, so we can see it in the viewport when it is driven. However, this may also be an invisble object, like an empty. Lets position it somewhere on the Z axis where it is somewhere which can't be seen during the cinematic sequence.

Next, we need to copy a new driver from the scene camera's focal length parameter.

Simply select the scene camera, go to the camera's Data tab where you find the Focal Length, and right click the parameter and select Copy as New Driver.

Now paste the driver onto a transform channel of the object that is to be driven, which in this case if the Cube that was made. This will result in the position of the cube to be the same value as the camera's focal length. In this case, 35mm focal length becomes 3500 in the X-axis.

If you don't like the extreme values, for instance due to fear of floating point errors, then you can make a custom expression by right clicking the transform channel where you pasted the driver and selecting Edit Driver. Here, you may change the type to Scripted Expression and write your own mathematical expression, for instance by dividing it by 10.

If you do so, then that must be accounted for in the Godot script below to get the right FOV during the sequence in Godot.

Now, we just need to export it.

About the script

A couple of things to look out for when using this script.

Firstly, on line 1 it says it extends a Node3D node. If you are using another type of node, then change the "Node3D" part to be the type of node your are using, or right click the node in the scene hierachy and select Change Type, and select Node3d.

Secondly, the paths for the camera and fov_reference_object must be correct. If you are unfamiliare with type node paths in Godot, your can have a look in the official documentation.

Other than that, I've added some comments in the script below for what's relative.

The script code

extends Node3D
 
@onready var camera = $Camera
@onready var animation_player = $AnimationPlayer
@onready var fov_reference_object = $FOV

#10 seems to result in a close replication of the Blender focal length. 
var sensor_width = 10
const pi = 3.14159

# Called when the node enters the scene tree for the first time.
func _ready():
    pass # Replace with function body.

# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
    var hfov_degrees = calculate_hfov(sensor_width, fov_reference_object.position.x)
    print(hfov_degrees)
    camera.fov = hfov_degrees
    pass

func calculate_hfov(sensor_width, focal_length):
    # Convert sensor width and focal length to radians
    var sensor_width_rad = sensor_width * (pi / 180)
    var focal_length_rad = focal_length * (pi / 180)

    # Calculate the horizontal field of view (HFOV) in radians
    var hfov_rad = 2 * atan(sensor_width_rad / focal_length_rad)

    # Convert HFOV from radians to degrees
    var hfov_deg = rad_to_deg(hfov_rad)

    return hfov_deg