getFocalLength() {

		const vExtentSlope = Math.tan( DEG2RAD * 0.5 * this.fov );

		return 0.5 * this.getFilmHeight() / vExtentSlope;

	}
Returns the focal length from the current {@link PerspectiveCamera#fov} and
{@link PerspectiveCamera#filmGauge}.

@return {number} The computed focal length.