def set_lens(focal_length, frame_height = 24.0)
      @fov = 2.0 * Math.rad_to_deg(Math.atan(frame_height / (focal_length * 2.0)))
      update_projection_matrix
    end
def set_lens(focal_length, frame_height = 24.0)
      @fov = 2.0 * Math.rad_to_deg(Math.atan(frame_height / (focal_length * 2.0)))
      update_projection_matrix
    end
Uses Focal Length (in mm) to estimate and set FOV
 35mm (fullframe) camera is used if frame size is not specified;
 Formula based on http://www.bobatkins.com/photography/technical/field_of_view.html