setFocalLength( focalLength ) {

		/** see {@link http://www.bobatkins.com/photography/technical/field_of_view.html} */
		const vExtentSlope = 0.5 * this.getFilmHeight() / focalLength;

		this.fov = RAD2DEG * 2 * Math.atan( vExtentSlope );
		this.updateProjectionMatrix();

	}
Sets the FOV by focal length in respect to the current {@link PerspectiveCamera#filmGauge}.

The default film gauge is 35, so that the focal length can be specified for
a 35mm (full frame) camera.

@param {number} focalLength - Values for focal length and film gauge must have the same unit.