Formula to convert After Effects to Three.js camera focal length

aeToThreeJSFocalLength.js

/* aeFocalLength must be in millimeters */


var getThreeFocalLength = function(viewportWidth, viewportHeight, aeFocalLength, filmWidth, filmHeight) {
     if ( filmWidth === undefined ) filmWidth = 36;
     if ( filmHeight === undefined ) filmHeight = 24;

     return (filmHeight*(viewportWidth / (filmWidth/aeFocalLength)))/viewportHeight;
}