How can orthographic to perspective camera?

[whilemouse]

Hi. I'm using your library very well. Thank you. :)
I asked about the contents of the previous similar mail.
How to change a perspective camera orthorgrapic camera.

Your responses

to 3D: PerspectiveCamera is possible.

const camera = new PerspectiveCamera( 20.0, canvas.width / canvas.height, 100, 5000 );
//You need to adjust modelZ accordingly, play around with this: let modelZ = 2725;

if(t3d.camera.isPerspectiveCamera) { 
    modelZ = (2725 / 480) * (canvasLayout.h);
} 

Best Regards Marcel

My question

1. Why fov is 20.0?

2. What does modelz's 2725, 480, canvasLayout.h mean and why?

3. perspective on the camera z axis fluid. 
   BRFFace does not return when I got a z axis, you can get this?